THE vertebrates: the amphibia and birds i6i 



account of endoderm formation (see p. 156) were accepted. There is little 

 doubt that in reptiles (p. 234) the endoderm comes from somewhere in this 

 general region of the embryo, and if in birds it originates from a defmite 

 part of the surface of the original cell-plate, this must be the place; but, 

 of course, if it arises entirely by delamination from the lower part of mass 

 of cells, there would be no definite location for it on a presumptive map 

 of the surface. 



We are on firmer ground when we turn to the other tissues. Vital 

 marks have clearly shown that most, if not all, the primitive streak and 

 the area on each side of it becomes mesoderm, while the area further 

 away takes part in the formation of the ectoderm ; the prospective neural 

 ectoderm lies anterior to the mesoderm near the midline. 



Comparing the maps of Amphibia and birds, one sees that their general 

 pattern would be similar if one might suppose that the amphibian gastrula 

 has been opened at some point within the area of skin ectoderm, and this 

 hole enlarged until the original map was flattened out to a circle. We 

 should then have an area of endoderm immediately round the blastopore, 

 surrounded by a ring of mesoderm fringing which is an outer ring of 

 ectoderm, with the neural ectoderm concentrated at one end, the anterior. 

 Tliis is exactly the picture we should find in the early primitive streak stage 

 of birds if we suppose that any endoderm originates from the surface. A 

 fuller discussion of the relations between the two groups is given on p. 243 . 



3. The gastrulation movements 

 (a) Amphibia 



We have now described the state of affairs at the outset of gastrulation 

 and given a sketch of the condition at the end of it. The process of gastru- 

 lation consists in the set of movements and foldings which convert the 

 former into the latter. It is clear that, since the bird and amphibian gastru- 

 lae differ so considerably, while the early embryos possess roughly similar 

 organs, the two processes must take different courses. These must now be 

 summarised. 



Gastrulation in the amphibian is the simpler of the two. It has been 

 followed in great detail by the vital staining technique, but for our 

 purposes it is only necessary to consider the general outlines of events. 

 Before the advent of the staining method, the gastrulation process could 

 only be inferred by comparison of a series of fixed and stained prepara- 

 tions of successive stages. It was natural to try to build up a picture in 

 terms of foldings, delaminations and localised growth; and a series of 

 technical terms, such as 'epiboly', 'emboly', 'invagination', 'involution', 

 etc. were employed in this connection. The newer methods showed that 



