124, SIDNEY F. HARMER. 
phore start ; and its position at the base of the aperture of the 
ovicell materially aids in the discovery of that structure in the 
series of sections. 
The secondary embryos do not lie freely in the cavity of the 
ovicell, but are still contained in a protoplasmic reticulum, 
which is the modified embryophore. The latter has largely 
increased in size with the commencement of embryonic fission, 
and has become branched, its lobes extending round the zoecia, 
which pass through the cavity of the ovicell (cf. fig. 6). The 
general appearance of a young embryophore in this stage in 
an entire colony is shown in fig. 12. 
The most striking difference between the development at 
this stage and that of Crisia consists in the absence of the 
primary embryo of the latter, and it often requires close exami- 
nation to prove that fission is still proceeding in Stage G. 
Such, however, is undoubtedly the case, although there is no 
coherent primary embryo left at this period. 
Fig. 37 illustrates the manner in which the number of second- 
ary embryos is increased during Stage G. An embryo similar 
to the smallest ones seen in fig. 11 is dividing transversely. 
The external and internal layers, which are so characteristic a 
feature of the Cyclostome embryo, are now well marked, and 
the inner layers of the two halves of the dividing embryo are 
completely separated from one another. The longest diameter 
of the embryo figured is 56 pn. 
In some colonies it is easy to demonstrate the occurrence of 
this process, and there are many cases where its recent occur- 
rence may be inferred from the fact of two small embryos lying 
close together in a position which suggests that they have 
recently separated from one another. It is obvious that if the 
embryo shown in fig. 37 had been cut transversely to its longest 
diameter it would have been by no means easy to be sure that 
fission was taking place. Another difficulty in proving the 
occurrence of this process is due to the fact that the normal 
development of a secondary embryo which is not going to 
divide further may, in section, appear very similar to the divid- 
ing embryos. The young definitive secondary embryo, which 
