562 



THE EMBRYOLOGY OF THE FROG 



as to reach the entoderm. This leaves a narrow, vertical ridge of cells 

 at the midline. 



A pair of slight depressions from the archenteron push upward and 

 appear only as virtual grooves formed by pigmented cells. Forward 

 from this, the lower margins of these grooves come to look like lips 

 which approach the midline. The mesoderm in the meantime separates 

 from the archenteron to a considerable extent (Fig. 324). 



Still farther forward, the grooves disappear, while the spaces be- 

 tween mesoderm and entoderm, as well as between mesoderm and ecto- 

 derm, extend vertically and delimit the pair of mesoderm masses. 



The cells thus left in the midline between the mesodermal sheets 

 form a wedge-shaped elevation which is continuous with the entoderm. 

 This is the beginning of the notochord. (Figs. 323, G, c, and 324, F, c,). 



The notochord is now cut off from the entoderm by a narrow slit 

 which leaves the archenteron with a dorsal roof, one cell in thickness. 

 (Fig. 324.) 



From this point posteriorly (toward the blastopore), the grooves 

 of the archenteron are better marked. 



THE MEDULLARY PLATE 



At the same time that the notochord is developing, the medullary 

 plate is also being formed (Figs. 324, np, and 326, everything above 

 mes.), partly from the medial band of cells which extends from the 

 region of the dorsal lip of the blastopore nearly to the animal pole, and 

 partially from the axial thickening caused by the confluence of the lateral 

 portions of the germ-ring. 



The inner portion of the former region is called the nervous layer 

 of ectoderm. It is this nervous layer which begins to thicken, and by 

 the time the blastopore has begun to close, a thickened medullary plate 

 has formed over the entire dorsal surface of the gastrula, essentially like 

 that described in the chick. 



At about the time the yolk-plug has disappeared, the lateral ridges 

 have become elevated and form the lateral neural ridges (Fig. 327, n, f). 



Fig. 326. 



Transverse section through frog tadpole in the an- 

 terior region of the neural groove, au, eye-pits. (Note the 

 pigment in the outer cells) ; ect, outer layer of ectoderm: 

 ent, entoderm ; mes, mesoderm ; nr. anlage of the neural 

 crest. (After Eyelesheymer. ) 



