214 RICHARD ASSHETON. 



it shows a slight advance. The light-coloured cells are smaller 

 and rather more numerous. 



In my next embryo, which is from a sheep of six days, the 

 inner mass of light-coloured cells is quite unmistakable. Its cells 

 number six. Of the dark outer layer there are now forty-six. 



As to the last three specimens, there can be no difficulty in 

 determining to which group any cell belongs, but from this 

 time forwards the work of tracing the lightly coloured cells is 

 far more intricate and the result less certain. I have tried to 

 represent in my figures as accurately as I can typical sections 

 of these stages. 



The all-important question to decide is, whereabouts in a 

 specimen such as those represented in figs. 10 and 11 does 

 the cavity of the blastodermic vesicle arise ? I have two 

 specim.ens in which this cavity is just beginning to be formed. 

 One was from a sheep of about six days ; of the age of the 

 other I have no record. My figs. 12 and 13 are both drawn 

 from sections of the former embryo. 



There is without doubt a marked difference in colour 

 between the several segments; but there is no longer a sharply 

 defined boundary between the dark cells and the light cells, as 

 there is in figs. 8, 9, and 10. And yet, on the whole, the light- 

 coloured segments are aggregated and pressed up against one 

 part of the wall of the vesicle (x) — a thin part of the wall — 

 much as they are in fig. 11, at the spot marked x. Amongst 

 the light cells are undoubtedly scattered darker ones. As an 

 explanation it may be said that, where the knife happened to 

 pass parallel to the surface of a cell boundary which becomes 

 included in the section resulting therefrom, darker areas will 

 appear among the lighter. 



In this specimen many strands may be seen passing between 

 the separating cells, which suggests that the cavity was only 

 just beginning to be formed at the moment of preservation. 

 It indicates also that there is protoplasmic connection between 

 at any rate certain of the segments at this time. 



Another specimen of unknown age, but seemingly a little in 

 advance of the one just described, shows similar features. 



