I i:\ BLOPM MI nvi> IMII.M \i;\ .I:I:M \..\\ 



89 



Avhole suri'ac of the _[. rm, with (he exception of a small place about 

 as lai'L'e M the head of a pin, which corresponds to the blast ,, 



appears black. Incau-" the small cells are deeply pigmented. \' 

 the place except ed a part of the impiumented y lk- M vudes. 



through the blast ojiore 

 and closes the entrance to 

 it as if \\ ith a stopper (d), 

 by reason of which it 



li .1 r.s i he Engnificanl name 



of viti lline plug. 



Of the two germ-layers 

 of the gastrula the outer 

 subsequent lv 1 M comes re- 

 duced in thickness in the 

 of the Water-Sala- 

 mander to a single layer 



Of r gularly a rrangcd Tigi 48 .-Sagittal section through an egg of Triton after 



cylindrical cells, when as the end of gastrulataon. 



, , at, ik, dz, dl, rt ud, as in fig. 47 ; d, vitelline plug ; 



111 the Case of the IrOg It mtf rai ddle germ-layer. 



is composed of tw r o or 



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

 The inner germ-layer in the roof of the ccelenteron likewise consi- 

 small (in tin- I-Yog, 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-ma s>. 

 1>. ing the Leavit r 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 cffilenteron, becomes the dorsum. The blastopore indicate 

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

 There may therefore be passed through the i:a>trula a longit i:dinal. 

 a dorso-vi ntral. and a transverse axis, which corn spond with the 

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

 so early in the Amphibia, is solely attributable to the 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 comprehension of th much 



