It is desirable only further to explain that gastrulation does not take place in all the Metazoa after exactly the same plan.
In most vertebrated animals this process of gastrulation has been more or less superseded by another, which is called delamination; but it scarcely seems necessary for our present purposes to describe the latter.
This proves the essential unity ofgastrulation in all the vertebrates, in spite of the striking differences in the various classes.
The gastrulation of the primitive fishes or selachii (sharks and rays) has been carefully studied of late years by Ruckert, Rabl, and H.
As the gastrulation of the amphioxus shows the original palingenetic form in its simplest features, that of the other vertebrates must have been derived from it.
I regard all those that diverge from the primary palingenetic gastrulation as secondary, modified, and cenogenetic.
Thus it was no light task to trace the identity of this process of gastrulation in all the animals.
We give the name of the gastrula to the important embryonic form that has these primitive organs, and the name of gastrulation to the formation of it.
This reduction becomes easier if, after considering thegastrulation of the tailless amphibia (frogs and toads), we glance for a moment at that of the tailed amphibia, the salamanders.
Both the segmentation of the ovum and the subsequent gastrulation have in this way been considerably changed.
It is to the influence of the different amounts of yolk present in the egg that are due the great differences in the segmentation and gastrulation processes, which almost mask their true significance.
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.
We have already seen that the parenchymula stage is passed over when the gastrulation is of the invaginate type.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "gastrulation" in a variety of sentences. We hope that you will now be able to make sentences using this word.
