The Development of the Chick 101 



and each germ-layer is made up of a single 

 layer of cylindrical cells. This is a simple and 

 typical form of gastrula. 



2. In some forms, the Amphibia, for ex- 

 ample, the large mass of yolk is accumulated 

 on the floor of the archenteron and reduces 

 that cavity to a narrow fissure. 



3. In Fishes, Reptiles, and Birds the egg is 

 large and contains a large amount of food 

 yolk. Since this yolk does not segment, it 

 can take no part in the process of invagination 

 which is confined, in consequence, to the 

 germinal disc. The yolk is very slowly en- 

 closed by a cellular wall, the ectoderm grow- 

 ing around it most rapidly, and the mesoderm 

 being the last to enclose it. 



4. In Mammals the egg is small, and the 

 inner germ-layer is derived from the thickened 

 region of the blastula, probably by a process 

 of invagination, since an aperture comparable 

 to the blastopore of Birds is seen at a later 

 stage. For a time the inner germ layer ends 

 freely below, so that the archenteron is closed 

 below by the ectoderm only, a condition compar- 

 able to that found in Birds, if we imagine the 

 yolk material to have been absorbed before 

 the completion of the middle germ-layer. 



