DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 143 



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

 two layers is the obliterated 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 kinds of 

 blastulse. 



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



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



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



accumulated on the ventral wall of the ccelenteron in 

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

 means of which the ccelenteron is reduced to a fissure. 



(c) In Fishes, Eeptiles, 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-disc 

 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 ccelen- 

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

 o ater 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 

 bilateral symmetry, so that one can easily distinguish the future 



