THE DEVELOPMENT OF THE CAPE CEPHALODISCUS. 199 
The cells of the ectoderm before fusion are arranged 
radially, and are at this stage clearly marked off from the 
inner cells ; a little later they are further marked off from 
them by the basement membrane. Soon after the fusion of 
these ectodermal cells a few cells appear below the basement 
membrane. These are closely applied to yolk granules, and 
form with them a distinct layer round the anterior end of 
the yolk mass, but clearly marked off from it (PI. 13, figs. 17, 
18 and 19, end.). As the yolk granules in this layer are 
used up, each of the cells sends out a long protoplasmic process 
towards the other, so that, ultimately, they form a chain of 
attenuated cells devoid of yolk granules. These cells appear 
first at the anterior end and later more posteriorly, so that 
the chain of cells gradually extends backwards, over the 
internal yolk mass, as an uninterrupted series, to the posterior 
end, at the point where the internal cells remain in connection 
with the ectoderm. 
Formation of the Yolk Cavity and Andersson's 
‘ ‘ Gastrula.” 
Meanwhile a change has taken place in the inner cells, 
associated perhaps with the appearance of the endoderm. 
Unlike the ectodermal cells, they do not assume a columnar 
form, but remain more or less rounded, each, however, with a 
nucleus and a distinct cell boundary, and as heavily laden 
with yolk granules as the ectodermal cells. On the appearance 
of the endodermal cells their cellular structure can no longer 
be distinguished. It appears as if the nuclei, with their 
associated protoplasm, no longer controlled these cells, and 
had wandered to the periphery, as in the case of the ecto- 
dermal cells, leaving a central non-cellular mass of yolk 
granules. 
All the cells of the inner mass do not pass to the periphery, 
but some find their way to the centre, where they form a 
small but distinct group, embedded in a substance nearly 
devoid of yolk granules. A small cavity then appears in 
