BKVELOPMENT OF THE TWO MIDDLE GEEM-LAYERS. 143 



inner germ-layer (ectoblast, entoblast) ; the fissure separating the- 

 two layers is the ohliterated cleavage-cavity; the cavity resulting 

 from the invagination is the ccelenteron, its external opening the 

 primitive mouth (blastopore, prostoma, crescentic groove, primitive- 

 groove). 



3. The four kinds of gastrulse correspond to the four Hnds of 

 blastulse. 



(a) In Amphioxus the coelenteron is -wide, and each germ- 

 layer is made up of a single sheet of cylindrical cells. 



(6) In Cyclostomes and Amphibia the mass of yolk-cells is 

 accumulated on the ventral wall of the coelenteron in 

 the inner germ-layer, and causes a protuberance, by 

 means of which the coelenteron is reduced to a fissure. 



(c) In Fishes, Reptiles, and Birds the process of invagination 



remains confined to the germ-disc, since the unsegmented 

 yolk, on account of its considerable volume, cannot be 

 made to share in the invagination. The germ-diso 

 becomes two-layered by means of an ingrowth of cells 

 at the crescentic groove (blastopore). The yolk acquires 

 a cellular boundary very slowly and at a late period ; 

 it is overgrown by the margin of the germ-disc, 

 when the supplementary cleavage (yolk-nuclei) takes 

 place. 



The outer germ-layer spreads itself out and envelops 

 the yolk most rapidly ; then follows the inner, and finally 

 the middle layer. 



(d) In Mammals the inner germ-layer is developed from the 



thickened region of the blastula, probably by means of 

 an invagination, because at a later stage an orifice of 

 invagination, comparable -with the primitive groove of 

 Birds, or a blastopore, can be demonstrated. At the 

 beginning of its development the inner germ-layer 

 terminates below in a free margin, so that the coelen- 

 teron is for a time closed in on the ventral side by the 

 outer germ-layer only, a peculiarity which is comparable 

 with the conditions in Reptiles and Birds, if we conceive 

 the yolk-material to have disappeared in this instance 

 before it is completely surrounded by the inner germ- 

 layer. 



4. In Vertebrates the gastrula presents a sharply expressed 

 Dilateral symmetry, so that one can easil distinguish the future 



