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 tho 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 it), and is 



distinguishable externally by means of 

 a sharp, afterwards horseshoe-shaped 

 furrow, which is bounded on one fide 

 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 ccelenteron (ud). 



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



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



however, it causes a com- 



plet' obliteration, of this 



cavity, the f unelus of the 



invagination becoming 



enlarged into a broad 



sac, while the entrance 



always remains narrow 



and fissure-like. Since 



the coelenteroii of the 



Amphibia was first ob- 

 served by the Italian 



investigator, RUSCONI, it 



is ordinarily mentioned 



in the older writings as 



RUSCONI'S digestive 



ud 



dl 



cavity, and the 



blasto- 

 the 



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 ; ud, coelenteron ; u, blastopore ; dz, yolk- 

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

 coelenteron. 



pore likewise as 

 RUSCONIAN anus. 



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



the vegetative half of the blast ula, has 1. -rn taken into the interior 

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

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



