88 



EMBRYOLOGY. 



\. 



Fig. 46. Egg of Triton, which is 

 developing into a gastrula, seen 

 from the surface. 



u, Primitive mouth (blastopore). 



constitutes the thinner roof, the latter the highly thickened floor, of 

 the excentrically placed cleavage-cavity. 



When the gastrula begins to be developed, the invagination 

 takes place on one side in the marginal zone (fig. 46 u), and is 



distinguishable externally by means of 

 a sharp, afterwards horseshoe-shaped 

 furrow, which is bounded on one side 

 by small cells, which in the case of 

 the Frog contain black pigment, on 

 the other side by large unpigmented 

 elements. At the fissure-like blasto- 

 pore there are infolded into the interior 

 of the blastula (fig. 47 u) along its 

 dorsal lip (dl) small cells, along its 

 ventral lip (vl) the large deutoplasmio 

 elements of the vegetative half; the 



former constitute the roof, the latter the floor, of the crelenteron (ud). 

 The latter appears in the first stages of the invagination simply as 

 a narrow fissure alongside the capacious cleavage-cavity (fh) ; soon, 

 however, it causes a com- 

 plete obliteration of this 

 cavity, the f undus of the 

 invagination becoming 

 enlarged into a broad 

 sac, while the entrance 

 always remains narrow 

 and fissure-like. Since 

 the co3lenteron of the 

 Amphibia was first ob- 

 served by the Italian 

 investigator, RUSCONI, it 

 is ordinarily mentioned 

 in the older writings as 

 RUSCONI'S digestive 

 cavity, and the blasto- 

 pore likewise as the 

 RUSCONIAN anus. 



At the close of the process of invagination the whole yolk-mass, or 

 the vegetative half of the blastula, has been taken into the interior 

 to form the lining of the ccelenteron, being at the same time over- 

 grown by a layer of small cells (fig. 48). Tn the case of the Frog the 



fit 



Fig. 47. Longitudinal [sagittal] section through an egg 

 of Triton at the beginning of gastrulation. 



ak, Outer germ-layer ; ik, inner germ-layer ; fh, cleavage- 

 cavity ; iid, coelenteron ; u, blastopore ; dz, yolk- 

 cells ; dl and vl, dorsal and ventral lips of the 

 coelenteron. 



