166 EICHARD ASSHETON. 



blast layers in the frog, it is best to examine the sections un- 

 stained. 



In Kana temporaria the epiblast is from a very early 

 period divided into two layers — an outer called the epidermic 

 layer, and an inner called the nervous layer. If a section is 

 taken, say, transversely though the neural plate of a tadpole 

 about the time of the folding up of the neural folds, a section 

 is obtained of which fig. 1 is a drawing. 



The two layers are seen to be very sharply and distinctly 

 divided, the outer or epidermic layer of epiblast (E. EP.) is a 

 single cell in thickness. The cells are much more deeply 

 pigmented than are the cells of the inner or nervous layer of 

 epiblast {E. NE.) which forms a much thicker layer. The 

 cells of this layer are very closely packed in the region of the 

 neural plate through which this section is taken ; so much so 

 as to render it impossible at this stage, at any rate, to distin- 

 guish the boundary of the cells — if, indeed, there are any 

 distinct boundaries. The nuclei are large, but are only seen 

 with difficulty without staining. Some indication of the 

 boundaries of the cells is to be seen in the slight increase of 

 pigment along certain lines. 



In fig. 2, which is taken at a slightly later stage, but before 

 the neural folds have completely closed, the cells of the 

 epidermic layer may be seen to have become much elongated, 

 their inner borders being no longer truncated, but mostly 

 pointed, and seem to be growing into the mass of nervous 

 epiblast. 



In fig. 3, which is from a section of a tadpole of about 

 3 mm. to 31 mm., in which the neural tube is now completely 

 closed and separated off from the skin, the appearance of the 

 darkly pigmented cells is very remarkable and instructive. 



I can see no reason to doubt that the darkly pigmented cells 

 of the epidermic epiblast, seen to be elongated in fig. 3, have 

 by this time elongated and passed right through the mass of 

 nervous cells (or nuclei) and spread out into fine filaments on 

 the further side of the nervous layer. I cannot say for certain 

 whether these fine filaments anastomose or not, but, however 



