EMBRYO FORMATION IN OTHER GROUPS OF VERTEBRATES 243 



homologise the structure of the blastoderm of the bird before endoderm 

 formation begins with the hollow sphere formed by the amphibian 

 egg at the end of cleavage; or one enquired how the chorda-mesodermal 

 canal in reptiles is related to the amphibian blastopore. Nowadays we 

 regard gastrulation as a co-ordinated series of foldings and movements 

 by which the various groups of cells which result from cleavage are 

 arranged into the three fundamental layers of the ectoderm, mesoderm, 

 endoderm. "What ought to be compared is the complete set of movements 

 in one form with the complete set in some other form, rather than any 

 particular instantaneous configurations which may be taken up during the 

 process. The most manageable way of summarising the whole gastrulation 

 of a particular group is provided by the map of presumptive areas at the 

 late blastula stage, together with an indication of the directions in which 

 these various areas will move. 



When one looks at the maps of presumptive areas which have been 

 described earher, it is fairly easy to arrange them into a scheme in which 

 they are brought into natural relations with one another. It is clear 

 that such a scheme should start from a type of egg which is fairly small 

 and has total cleavage, since the blastodermic forms of development must 

 be secondary derivatives. Of the totally cleaving eggs, the most primitive 

 fish (cyclostomes) and the Amphibia have maps which greatly resemble 

 one another. The blastopore is placed low down on the vegetative side 

 within an area of endoderm. Above this is a ring of presumptive meso- 

 derm, wliich, certainly in the Amphibia and probably in the cyclostomes, 

 extends right round the egg from the dorsal to the ventral side. Within 

 this ring the presumptive axial mesoderm (notochord and somites) is con- 

 centrated somewhat towards the dorsal side, but extends much further 

 laterally in the blastula than it will do after gastrulation is completed. 

 The upper part of the egg is occupied by a cap of presumptive ectoderm, 

 of which the region near the dorsal plane will become neural plate and the 

 remainder epidermis. A very similar pattern is found in the protocordates 

 (ascidians and Amphioxus), and we are probably safe in taking this as the 

 basic type from which the others should be derived (Fig. ii.io). 



There are two other main types to be fitted in. Between the cyclo- 

 stomes and the Amphibia in the evolutionary sequence come the 

 cartilaginous and bony fishes. These have a blastodermic type of develop- 

 ment, and a characteristic feature of their presumptive map is that the 

 margin of the blastoderm is made of presumptive endoderm or possibly 

 mesoderm; the whole ectodermal area lies well inside the margin. The 

 other groups — reptiles, birds and mammals — are evolutionarily more 

 advanced than the Amphibia. Their embryos also develop from a 



