THE DEVELOPMENT OF THE CAPE CEPHALODISCUS. 201 
It appears from what lias been observed that, in this species, 
the central yolk-laden cells arise solely by unipolar prolifera- 
tion of cells into the cavity of the blast ula, that the cellular 
structure breaks down, and some of the nuclei, with their 
associated protoplasm, goto form the endoderm, while others 
pass towards the centre and become vitellopliags. Owing 
to the activity of these latter the yolk granules become 
used up, leaving a homogeneous detritus in which a cavity 
subsequently appears. This cavity extends at first to an 
•ectodermal involution, and it may be that the excretory 
matter passes out in this way to the exterior, just as the 
•excretory products of the growing ectoderm are given off in 
another manner already indicated. The subsequent changes 
•in this cavity and its relation to the posterior involution, as 
well as to the cavity immediately enclosed by the endoderm, 
will be described in later stages. 
Formation of Internal Yolk Columns. 
After the inner yolk-laden cells become a homogeneous 
mass of yolk with scattered cells, and soon after these 
reduced cells begiu to migrate, some towards the periphery, 
some to the centre, a change takes place in the uniform 
distribution of the yolk granules, and they assume the form 
of a number of yolk columns, or rather pyramids, whose 
apices meet round the central cavity, and whose broader 
distal extremities are in the proximity of the cells which form 
the endoderm (PI. 13, figs. 18 and 19, i. y. c.). The result 
bears some resemblance to what has occurred in the ectoderm, 
but it has apparently been attained in a different way, for 
the large yolk-laden cells do not individually become yolk 
columns ; at least there was no appearance of this, and it 
can hardly be imagined how they could do so, unless perhaps 
the cells, 'migrating outwards to form the endoderm, retained 
for a time some control over their original yolk masses, and 
similarly the cells migrating inwards draw out their associated 
yolk into lenticular masses. 
