168 RICHARD ASSHETON. 



superficial layer covering the upper pole. Hence for a long 

 time we find the superficial layer of cells after segmentation 

 to be more deeply pigmented than the more internally situated 

 segments. 



Secondly. Pigment seems to be in some way connected with 

 the actual protoplasmic activity, as it appears internally wher- 

 ever division of cells takes place. 



So also I believe the intensely black appearance of the pro- 

 cesses of the epidermic epiblast which I have been describing 

 is in some way connected with their intense activity just evinced 

 by their growth inwards — which seems to be very rapid, and 

 rather sudden. 



When at a later period certain groups of nerve cells show a 

 similar intense activity, there is a similar deposition of pigment 

 in and around their processes, as, for instance, in the develop- 

 ment of the ganglion habenula as shown in fig. 5. This pig- 

 ment in each case becomes greatly lessened after the period of 

 intense activity of growth has passed by. 



The Question of Early Separation into Neurohlastic 

 and Spongioblastic Elements. 



Although the evidence to be drawn from the figures accom- 

 panying this paper is far from being conclusive, yet I think 

 it tends very strongly towards the inference that the epidermic 

 layer of epiblast in the frog gives rise to the spongioblastic 

 elements, and the nervous layer to the neuroblasts. 



His has shown how the spongioblastic network precedes the 

 development of neuroblasts and nerve fibres and forms an 

 irregular network with angular processes by no means unlike 

 the dark strands in my fig. 3. These processes are not at all 

 like the processes of nerve cells, which are always more 

 tapering and less knotty. Further, at this stage there cannot be 

 found any trace of definite nerve-fibres. These do not appear until 

 later, till the tadpole has attained a length of about 6 — 6h mm. 



I can only interpret these figures as showing the con- 

 version of the epidermic layer of epiblast into a supporting 



