So in the vertebrate eye most observers speak only of a thickening to form the retina, but Goette's observation points to an invagination of the optic plate at an early stage.
All the known cases of inversion among arthropods occur in eyes, the retina of which is simple, and are all natural consequences of the process of invagination by which {114}the retina is formed.
Finally, in certain Echinodermata the ventral part of the central nervous system arises by the invagination of a linear streak of ectoderm, the cavity of the invagination persisting as the epineural canal.
There are two main methods in which the first two are differentiated--invagination and delamination.
In Peripatus the cerebral ganglia arise in a similar way, but in this case the cavities of the invagination become separated from the skin and persist as two hollow appendages on the lower side of the cerebral ganglia.
The difference is evidently entirely due to the presence of yolk, and the consequent modification of invagination in the former case.
When the surrounding solution is altered in constitution or when the temperature is too high, the intestine instead of growing into the hollow sphere grows outside, we get an evagination instead of an invagination of the intestine.
In the majority of the Decapoda there is a saccular invagination of the integument in the basal segment of the antennular peduncle having on its inner surface "auditory" setae of the type just described.
This depression increases in depth until a distinct anal invagination results, known as the proctodaeum, which grows as a funnel-shaped fossa toward the blind termination of the endgut.
The external perineal folds separate the anal invagination from the uro-genital opening.
It is derived from the ectoderm and its production is analogous to the formation of the oral cavity by means of the ectodermal invagination called the stomadaeum.
Seessel's sac is the cephalic blind termination of the embryonic foregut before the communication with the ectodermal invagination of the stomadaeum has been formed.
The embryo now consists of two layers of cells, epiblast and hypoblast, surrounding a cavity, the archenteron, which opens to the exterior by the orifice of invagination or blastopore.
I have further satisfied myself as to the absence of such an invagination in Peripatus.
The growth outwards of the mesoblast from the axis of the primitive streak is probably a remnant of the invagination of the hypoblast and mesoblast from the lip of the blastopore in Amphibia, &c.
After the larva has for some time enjoyed a free existence, a remarkable series of changes takes place, which result in the invagination of the half of it formed of the clear cells, and form a prelude to the permanent attachment of the larva.
In these two considerations there may, perhaps, be found a sufficient explanation of the invagination of the ciliated cells, and the growth of the amoeboid cells over them.
This difficulty is the invagination of the ciliated cells instead of the granular ones.
As a result of this invagination the segmentation cavity is obliterated and the larva assumes a compressed plano-convex form with a central gastrula cavity, and a blastopore in the middle of the flattened surface.
The entire process ofinvagination is completed in about half an hour.
The first part of the urinogenital system to develop is the segmental duct (Vornieregang of Fuerbringer), which is formed by a groove-like invagination of the peritoneal epithelium.
At the narrow end an invagination takes place, which gives rise to an open sac, the blind end of which is directed forwards.
It is probable that Hatschek and other observers who have followed him are mistaken in affirming the existence of such an invagination in either the Chaetopoda or the Arthropoda.
It is distinguished from the stomod, a part at the anterior end of the canal, including the cavity of the mouth, and the proctod, a part at the posterior end, which are formed byinvagination and are lined with epiblast.
This invagination or bulging-in obliterates the original inner cavity of the Blastula.
In Lepidosteus the air-bladder appears to arise, as in the Teleostei, as an invagination of the dorsal wall of the oesophagus.
The lens is formed as an invagination of the single-layered epidermis (fig.
The invagination of the mesenteron has in the meantime extended very far forwards, and the segmentation cavity has become obliterated.
In Amphioxus, where the small amount of food-yolk present is distributed uniformly, there is no reason why the invagination and resulting gastrula should not be symmetrical.
Owing in fact to the presence of the food-yolk it is a mixture of invagination by epibole and by embole.
This epiblastic invagination is the proctodaeum, and an anal perforation eventually appears at its upper extremity.
The roof of the mesenteron is formed by the continued invagination of small cells, and its floor is composed of large yolk-cells.
On purely a priori grounds there is in my opinion more to be said for invagination than for any other view.
At the inflected edge an invagination of cells takes place, underneath the epiblast, towards the segmentation cavity, and gives rise to the dorsal wall of the mesenteron and the main part of the dorsal mesoblast.
Before the invagination is completed the larva throws off the egg-membrane, and commences to lead a free existence.
Firstly it is extremely doubtful whether the invagination of the secondary optic vesicle is to be viewed as an actual mechanical result of the ingrowth of the lens.
In Teleostei the auditory pit arises as a solidinvagination of the epiblast.
This is a process of gastrulation by invagination which is found in all animals above the Coelenterata, but which is very rare in the Cnidaria, and is known only in the Scyphomedusae amongst the Hydrozoa.
The orifice of invagination forms a pore which may be closed up or may form a protruding duct or funnel.
The planula becomes elongated and broader towards one pole, at which a pit or invagination of the ectoderm arises.
Hence the cavity of the air-sack is equivalent to a sub-umbral cavity in which no manubrium is formed, and the pore or orifice of invagination would represent the margin of the umbrella.
The part of the cloaca from the upper wall of which it forms belongs to the proctodaeum, the ectodermicinvagination of the rectum (Chapter 2.
In the end this invagination goes so far that the outer or invaginated part of the blastoderm lies close on the inner or non-invaginated part (Figure 1.
Hence before and during invagination there is an increasing differentiation of the inner from the outer layer.
There is, however, an invagination of the ectoderm at the mouth, which has given rise to a muscular pharynx (sd).
This is done in just the same way as the invagination of the blastula, which gives rise to the gastrula in the amphioxus (Figure 2.
Attempts to plug the canal have, in most cases, been made by invagination of the skin of the scrotum and its fascia.
The length of the invagination varies from two or three inches to three, four, or five feet.
It is doubtful whether there is on record a single well-authenticated case of inflammatory invagination where the lower segment of bowel protruded into the upper.
When invagination involves the upper part of the small intestine, diagnosis of the cause of occlusion is almost impracticable.
At first the invagination involves a small portion of the bowel, but, active peristaltic action continuing, it rapidly increases in size.
There is no doubt that the continuity of the intestine above and below the neck of the invagination has been established, and complete cures effected in the way already mentioned.
At first pressure gives relief, but in a few hours tenderness, denoting peritonitis, appears, limited to the invagination or spreading gradually over the whole abdomen.
Generally the tumor formed by the invagination can be felt through the abdominal wall, and is a symptom of great importance.
One of the most frequent and important causes of intestinal obstruction is intussusception or invagination of the bowel; by which term is meant the protrusion or slipping of one portion of bowel into a portion immediately adjoining.
The urgency of the symptoms of invagination depends upon the portion of bowel involved and the degree of constriction of the gut and its attached mesentery.
Thin sections showed that these processes were formed by invagination of the test beneath the dorsal and glabellar furrows.
Its anterior portion or pharynx, which arises from the stomodaeal invagination in the embryo, is short; a pair of large, so-called salivary glands open into it.
It results from this arrangement that the cells forming all but the bottom of the invagination lie horizontally, i.
C, The primitive invagination to form one of the otocysts, as seen in fig.
The "white body" of the adult ocular capsule forming as an invagination of the outer cell-layer.
An invagination is next formed on the oral surface, which gives rise to a sack opening to the exterior (fig.
He finds that it develops as a paired invagination of the epiblast immediately ventral to the anus.
At a somewhat later period aninvagination of the epiblast grows into each of these lobes.
A normal invagination obliterating the segmentation cavity then ensues, and the blastopore narrows to form the mouth.
The oral invagination appears nearly coincidently with the first formation of segments at the front end of the groove between the lateral nerve cords, and the anal invagination appears slightly later at the hindermost end of the ventral plate.
Before the invagination of the hypoblast is completed the mesoblast makes its appearance in the form of two bands or streaks, extending longitudinally for the whole length of the embryo.
After the larva has for some time enjoyed a free existence, a remarkable series of changes take place, which result in the invagination of the half of it formed of the clear cells, and form a prelude to the permanent attachment of the larva.
Bobretzky first stated that the invagination remained open, but subsequently corrected himself.
This invagination is compared by Hoek to the invagination in Astacus, and is believed by him to give rise to the mesenteron.
In Lumbricus and Criodrilus the blastopore remains as the mouth, but in Euaxes a new mouth or rather stomodaeum is formed by an epiblastic invagination between the front end of the two mesoblastic bands.
An invagination appears, usually on the right in Gasteropods, and eventually extends to the dorsal side (fig.
The oesophagus is derived from an invagination of the epiblast, and the remainder of the alimentary canal from the archenteron.
The little mass of hypoblast or enteric cell-mass now enlarges, but remains connected with the cicatrix of the blastopore or orifice of invagination by a stalk, the rectal peduncle.
The organs are developed as invaginations of the epidermis of the foot, and in the majority of the Protobranchia the orifice of invagination remains open throughout life; this is also the case in Mytilus including the common mussel.
A cavity formed by theinvagination of the outer wall of the body, as the atrium of an amphioxus and possibly the body cavity of vertebrates.
Further cell-divisions occur, and the invagination becomes deeper, until the invaginating wall nearly touches the wall which has retained its primitive position.
Local conditions of growth cause the invagination of a set of the cells of the blastosphere-wall.
Foregut: extends from the mouth to the end of gizzard; its epithelium being formed from the ectodermal invagination known as the stomodaeum.
Endomesoderm: the inner layer formed by an invagination of the middle portion of the primitive band of the embryo, and from which the endoderm and mesoderm are subsequently differentiated.
Thus the gastrulation of the placentals, which diverges most from that of the amphioxus, the primitive form, is reduced to the original type, the invagination of a modified blastula.
When the blastula is fully formed, we have again in this case the important folding or invagination that determines gastrulation.
This invagination or folding proceeds from the primitive mouth, at the two sides of which (right and left) a couple of pouches are formed.
The invagination or the folding inwards of the bird-blastula takes place in this case also at the hinder pole of the subsequent chief axis, in the middle of the hind border of the round germinal disk (Figure 1.
In the middle of its hinder border we have the beginning of the invagination of the primitive gut (Figure 1.
But in the third sub-class of the mammals the serolemma forms, by invagination at its outer surface, a number of hollow tufts or villi, from which it takes the name of the chorion or mallochorion.
A very important and remarkable process now follows--namely, the curving or invagination of the blastula (Figure H).
D, E); it is most conspicuous when the invagination is complete and the segmentation-cavity has disappeared (Figure 1.
Nevertheless, the gastrula is formed here, as in the previous cases, by the folding or invagination of the blastula.
The tip of the latter appears to be in direct connection with the nuclear membrane at the centre of the base of the flask-shaped invagination (figs.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "invagination" in a variety of sentences. We hope that you will now be able to make sentences using this word. Other words: introversion; retroflexion; reversal; reversion; revulsion; transposition
