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cell. This spot can be recognised in later stnges, and will 
be denominated a’. In the embryo under consideration a 
cell can be seen at this point whose nucleus exhibits dot-like 
aggreg'ations of chromatin, indicative of one of the prophases 
of mitosis. Another case of mitosis can be seen in the middle 
of the convex surface. When we examine the lower border of 
the section at the point marked y in the figure where the 
convex surface passes into the flat surface, we find that there 
is a gradual transition from the rectangular to the columnar 
type of cell. If wm now examine the nuclei we find that in 
all the cells they are of a uniform character so far as their 
staining properties are concerned. They present a vesicular 
appearance, the nuclear sap being crossed wnth cords of liuin 
on which are very minute chromatin granules. Turning our 
attention now to the grannies of yolk we find that these are 
most numerous in the large cells of the flat side and become 
fewer as we pass to the cells of the convex side, and that 
there appear in these latter cells smaller granules amongst the 
larger ones, but that there is no difference whatever either in 
size or in staining properties between the largest yolk-granules 
in the cells of the convex side of the blastula and those in 
the cells on the flat side. In this stage of development, there- 
fore, it is impossible to say that the cells of the embryo are 
segregated into two definite tissues, as Lwoff and Cerfontaiue 
have maintained, for as we have already remarked, and as a 
simple inspection of the figure will prove, the cells of the flat 
side pass by insensible gradations into those of the convex 
side. If with these authors we call the cells of the convex 
side ectoderm and those of the flat side eudoderm, it is im- 
possible, if we consider the point y, to say where the one 
ends and the other begins. At the point x there is an abrupt 
change in the size of the cells, and this is manifestly due to 
the beginning of a process of division by which the size of 
the cells is reduced. The smaller yolk-granules which appear 
amongst the larger ones in the cells of the convex side are 
due to the utilisation and fragmentation of these owing to 
the process of multiplication of cells which has just taken 
