116 



EMBRYOLOGY. 



dinal fissure. A cross section made through the middle of the 

 embryo in front of the blastopore (fig. 77) corresponds in every 

 particular to our first diagram (fig. 73), if we conceive that the 

 body-cavity in this case has disappeared. The outer germ-layer (ak) 

 consists of a single sheet of cells, which on the back of the embryo 

 are cylindrical, but become shorter toward its ventral side. The 

 cells enclosed within the outer layer exhibit a differentiation in three 

 ways, and therefore are subsequently converted into three different 



Fig. 76. 'Egg of Triton with distinctly developed medullary groove, seen from the blastopore, 



53 hours after artificial fertilisation. 

 D, Dorsal, V, ventral region ; u, blastopore ; h, elevation between blastopore and medullary 



groove (r) f, semicircular furrow, which encloses the blastoporal area ; dp, yolk-plug. 



Fig. 77. Cross section of an egg of Triton with feebly expressed medullary groove. 

 ak, Outer, ik, inner germ-layer ; mk l , parietal, mir", visceral lamella of the middle germ-layer ; 

 ch, chorda ; dh, intestinal cavity ; D, dorsal, V, ventral. 



organs into chorda, entoderm, and middle germ-layer. First, there 

 is to be found on the roof of the ccelenteron (dh) under the medullary 

 groove, even close up to the blastopore, a narrow band of long 

 cylindrical cells (ch) ; it corresponds in every respect to the funda- 

 ment of the chorda in our diagram (fig. 73 ch), and in the cross 

 section through Amphioxus (fig. 74 ch). Secondly, the fundament 

 of the chorda is flanked on either side by two bands (mk l , mk*) of 

 small oval cells, which extend downwards to 'Vhxnit the middle 

 of the lateral region of the embryo. They do not share in bounding 

 the coelenteron, since a third kind of cells (ik), large and rich in yolk, 

 lie along their inner surfaces. The latter begin at the margin of 



