Now in these groups no zoologist would deny the homology of the mouth and anus in the different forms, and yet how very different is their history even in closely allied animals.
Here no one would doubt the homology of the mouth and anus throughout the group; yet within the limits of a single genus--Peripatus--they show the most diverse modes of development.
Embryology thus confirms the results of Comparative Anatomy in reference to the homology of these ganglia in the two groups.
The chief difficulty in this connection is the homology of the third pair of post-oral appendages.
A micropyle is by no means found in the majority of types, and there is no homology between the various apertures so named.
The epiblastic invaginations are represented in it very accurately, and though not mentioned in the text of the paper, Moseley informs me that he has long been aware of the homology of these folds with those in various other Tracheata.
The arguments which can be urged for the opposite view do not appear to me to have much weight, so that the homology of the appendages in question with the second pair of maxillae may be provisionally assumed.
S, Blood-spaces, of which the homology is still uncertain.
So also are the maxillæ and labium, though we are not aware that any one has indicated how close the homology is.
Footnote 10: The directhomology of these parts of the head (the occiput and the epicranium) with Perla, Forficula, etc.
Relating to that kind of homology or symmetry, the mathematical conception of organic form, in which all axes are equal.
Homology indicates genetic relationship, and according to Haeckel special homology should be defined in terms of identity of embryonic origin.
The homology of parts arranged on transverse axes.
We do not here seek for similarity of form, but homology of regions; and it is only by using the same names to denote the same things that we can succeed in determining such homology.
The action of the anterior part being clearly comparable to the human great gluteal, there can be no doubt as regards the homology of this portion, so we will not insist on it further.
Homology of names should, in our opinion, always accompany homology of regions.
Dear Professor Pelseneer, I am very glad to hear from you that the homology of the cephalopod arms with the gasteropod foot is now generally admitted.
According to this principle, the homology of a part is determined by its position relative to other parts.
Homology is still determined by position, by connections, in the embryo as in the adult.
But we have had to do only with a modification, not with a transformation, of the criterion of homology recognised by the anatomists.
Special homology is the name we give "to the relations which obtain between two organs which have had a common origin, and which have also a common embryonic history" (Elements, p.
The germ-layer theory was found inadequate to the new facts, and many reverted to the older criterion of homology based on destiny rather than origin.
He admits that there exists a real homology between, for instance, the notochord of Vertebrates and that of Ascidians.
One of these comparisons is, however, a homology not an analogy, and the last phrase throws a little doubt upon the whole question, for it is not made clear whether it is position or function that determines what are equivalent organs.
Conversely, the same complex of materials, say a fore limb, may take on the most varied shapes according as the function of the organ changes--but homology remains though form changes.
Such is, in my opinion, the nature of the homology existing between the tail of the ascidian tadpole and that of Amphioxus or of young amphibians.
Homology has reference to community of origin, however obscured to the superficial observer such common origin may be by modifications necessary to adapt to different functions.
Now, as already stated, homology can not be traced with any certainty between the primary groups, but within the limits of each group it may be traced with ease and beauty.
Undoubtedly the true principle on which primary groups ought to be made is, identity of general plan of structure, or traceableness of homology throughout.
In other words, it is impossible, in the present state of knowledge, to trace homology with any certainty from one group to another.
In the visceral organs homology is very plain, in fact too plain.
But within the limits of each primary group the homology is easy.
This completely explains why it is that we are able to trace homology only within the limits of each primary group.
Of the homology of fore and hind limbs and consequently of their like origin there can be no doubt.
The problems of analogous variation or parallelism without homology are very often met with among fishes.
The brain is composed of paired ganglia in general appearance resembling the brain of the true fish, but the detailed homology of its different parts offers considerable uncertainty.
But this homology does not extend to the details of structure.
The homology of the air-bladder with the lung is evident.
Moreover, no thought of the geological past of groups and species had yet arisen, and without the conception of common origin, the facts of homology had no significance.
The turtle-like mandibles seem to be formed of dermal elements, in which there lies little homology to the jaws of a fish and not much more with the jaws of Dipnoan or shark.
Therefore there is an a priori improbability against the homology with the scapula of any part having a distant and merely ligamentous connection with the humerus-bearing element.
Dean remarks: "The entire problem of the homology of the dermal plates and 'scales' in the Ostracophores and Arthrognaths is to the writer by no means as clear as previous writers have conceded.
It is not certain that fishes possess a sense of taste, and it is attributed to them only through their homology with the higher animals.
Zittel leaves them with the Ganoids, where Agassiz early placed them, but they show little homology in structure with the true Ganoids.
In this view there is no real homology between the archipterygium in the sharks possessing it and that in the Dipnoans and Crossopterygians.
Analogy is the correlative of physiology, homology of morphology; but the two may be coincident, as when structures identical in morphology are used for the same purposes, and are therefore physiologically identical.
Before proceeding with it I should say that I assume the serial homology of fore and hind limbs to be beyond dispute.
But in spite of its resemblance to a fin, the spine in Pterichthyodes is an outgrowth of the ossified skin and has no more homology with the spines of fishes than the mailed plates have with the bones of a fish's cranium.
The great facts of homology were clearly understood by Cuvier.
Everywhere we find the distinction between homology and analogy which was explained in the last chapter--the distinction, that is, between correspondence of structure and correspondence of function.
The above is the view commonly accepted concerning the homology of the carpal and tarsal bones.
This makes it probable that the so-called ilium of the Frog is really to be regarded as an ilio-pubis, and renders the homology of the cartilaginous part uncertain, but it probably corresponds to the acetabular bone of mammals.
I rather doubt whether you see how far, as it seems to me, the argument for homology and embryology may be carried.
He will surely come to this from Homology and Embryology.
Defn: That form of homology due to common ancestry (phylogenetic homology), in opposition to homomorphy, to which genealogic basis is wanting.
Note: Homology indicates genetic relationship, and according to Haeckel special homology should be defined in terms of identity of embryonic origin.
Defn: The homology of parts arranged on transverse axes.
In an extended sense, the term is applied to the relation between chemical elements of the same group; as, chlorine, bromine, and iodine are said to be in homology with each other.
Defn: Relating to that kind of homology or symmetry, the mathematical conception of organic form, in which all axes are equal.
They are, however, more superficial in position, and this has led many to doubt their actual homology with the cartilaginous visceral arches.
That there is a general homology appears on the other hand hardly open to doubt.
To what extent is there a complete homology between the two primary germinal layers throughout the Metazoa?
It is however quite possible, though far from certain, that the ureter of Amniota may be a special formation confined to that group, and this fact would in no wise militate against the homology I have been attempting to establish.
The question as to how far there is a complete homology between the two primary germinal layers throughout the Metazoa was the third of the questions proposed to be discussed here.
I am not inclined to think that there is a true homology between these organs and the lateral line of Vertebrata.
These great differences in development are important in reference to the homology of the metanephros or permanent kidney, which is discussed below.
A question which has to be answered in reference to the segmental tubes is that of the homology of the secondarily developed peritoneal openings of Amphibia, with the primary openings of the Elasmobranchii.
These facts are important to remember, since we shall presently perceive, upon analyzing their parts, that these Hydrae have a close homology to the Hybocodon.
In the account of the Nanomia, the homologybetween its scale, or abortive Medusa, and the Hybocodon, is traced in detail, and I need only allude to it here.
In spite of the apparent degeneration of the first two pairs of appendages in Calymene, no limbs are actually missing, and if some are dropped out in the later trilobites it would not affect the homology of those now known.
There seems to be some doubt about the homology of the pedipalps with the mandibles, as at least one investigator claims to have found rudiments of a segment between the one bearing the chelicerae and that with the pedipalps.
In this he was obviously incorrect, but it accounts for his homology of the remainder of the cephalic appendages.
No homology, if homology is intended, could be more erroneous.
The homology of the third post-oral limb is in question, some considering it a maxilla and others a maxilliped.
If the two terms were interchanged, the homology with the limb of the trilobite or other crustacean would be quite perfect.
A great deal of Doctor Walcott's difficulty probably arises from his homology of the coxopodite of the trilobite with the protopodite of the higher Crustacea.
They are the homology of the organs, and to a certain degree also the so-called rudimentary organs.
If this is so, then this homology ought to throw light on the extraordinary innervation of these tubular muscles by the branchialis profundus branch of the facial nerve and the velar branch of the trigeminal.
But there is one most important fact which ought to make us hesitate before accepting the homology of the dorsal heart of the arthropod with the ventral heart of the vertebrate--The heart in all invertebrates is a systemic heart, i.
The homology of the germinal layers constitutes in all cases of disputed relationship the court of final appeal.
Limulus appendage, especially when the homology of the Ammocoetes branchial cartilage with the Limulus branchial cartilage is borne in mind.
At present the homology suggested is not complete, for there is no evidence as yet that the veno-pericardial muscles have anything to do with the pericardial nerve.
Yet so clear is the evidence that it is difficult to see how the homology can be denied.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "homology" in a variety of sentences. We hope that you will now be able to make sentences using this word.
