FUSION OF EPIBLASTIC LAYEES IN BABBIT AND FROG. 169 



framework in between the cells of the nervous layer. It 

 is quite possible that we ought to regard the nervous layer of 

 epiblast as comparable to the germinal cells of His rather than 

 to the fully-developed neuroblasts. 



If this is correct, we then must conclude that in the frog 

 there is a very early separation of the neuroblastic from the 

 spongioblastic elements. 



Comparison between Rabbit and Frog. 



Is it then possible that the condition in the rabbit has its 

 parallel here in the frog ? It is true that the epiblast is 

 double only over a certain area of the embryo in the rabbit, 

 whereas in the frog it is double throughout. 



In the frog the nervous layer soon becomes much thickened 

 along the future dorsal surface of the embryo, and over the 

 rest of the embryo the nervous layer becomes reduced to a 

 layer of one cell only in thickness, like the epidermic layer. 



Now, although it is extremely difficult to trace the history 

 exactly, I am almost sure that the area over which the inner 

 layer of epiblast cells in the rabbit is found, corresponds to 

 that area in the frog over which the nervous epiblast remains 

 thick, or becomes thicker — i. e. to the neural plate. 



In the frog the whole of the neural plate does not become 

 folded up to form the neural tube, but the outer lateral portions 

 of the anterior part remain outside of the tube, giving rise to 

 the ganglia of the anterior cranial nerves. 



I have endeavoured elsewhere to bring evidence to show that 

 the epiblastic wall of the anterior part of the body of the 

 rabbit embryo includes more than the double-layered part of 

 the embryo, i. e. more than the so-called " embryonic disc." 



Whether the " embryonic disc " goes to form the neural tube 

 and ganglia of anterior cranial nerves as in the frog, or whether 

 it forms only the neural tube, I have no evidence to oflPer. 



The embryonic disc is precisely in the same position relative 

 to the primitive streak as is the neural plate to the blastopore 

 of the frog. 



I am well aware that the epiblast is not at first double in all 



