DEVELOPMENT OF THE TWO PRIMARY GERM LAYERS. 



89 



whole surface of the germ, with the exception of a small place about 

 as large as the head of a pin, which corresponds to the blastopore, 

 now appears black, because the small cells are deeply pigmented. At 

 the place excepted a part of the unpigmented yolk-mass protrudes 

 through the blastopore 

 and closes the entrance to 

 it as if with a stopper (d), 

 by reason of which it 

 bears the significant name 

 of vitclline plug. 



Of the two germ-layers 

 of the gastrula tne outer 

 subsequently becomes re- 

 duced in thickness in the 

 case of the Water-Sala- 

 mander to a single layer 

 of regularly arranged 

 cylindrical cells, whereas 

 in the case of the Frog it 

 is composed of two or 



three layers of small, in part cubical, deeply pigmented elements. 

 The inner germ-layer in the roof of the cceleiiteron likewise consists of 



Fig. 48. Sagittal section through an egg of Triton after 



the end of gastrulation. 

 ak, iic, dz, di, vl, ud, as in fig. 47 J d, vitelline plug ; 



mk, middle germ-layer. 



small (in the Frog, pigmented) cells, but in the floor it is composed 

 of large yolk-cells, which, heaped together in many layers, pro- 

 duce an elevation that projects far into the ccelenteron and partly 

 fills it. For this reason the gastrula in Amphibia is compelled 

 to adopt in water a definite position of rest, because the yolk-mass, 

 being the heavier part, always assumes the lowest position (fig. 48). 



The germ of the Amphibia is already a bilaterally symmetrical 

 body. The thickened, yolk-containing wall of the gastrula becomes 

 the ventral side of the adult animal ; the opposite wall, or roof of 

 the ccelenteron, becomes the dorsum. The blastopore indicates, as 

 the sequel shows, the posterior end, the opposite part the head-end. 

 There may therefore be passed through the gastrula a longitudinal, 

 a dorso- ventral, and a transverse axis, which correspond with the 

 axes of the adult animal. This bilateral symmetry, which appears 

 so early in the Amphibia, is solely attributable to tne accumulation 

 of yolk- material, and to the piling up of it on the ventral side of the 

 ccelenteron. 



The development of Amphibia furnishes us with a transitional 

 condition, which is serviceable for the (rmnpivhonsion of the much 



