EMBRYONIC FISSION IN CTOLOSTOMATOUS POLYZOA. 205 



closed by a chitinous uncalcified membrane (ectocyst). This 

 is the condition of the ovicell, and of the growing-points 

 generally, at all stages before their growth is completed and 

 the definitive apertures are formed. The funnel, which is, in 

 fig. 19, the most conspicuous part of the ovicell, is consequently 

 merely that part of the ovicell in which calcification has 

 occurred. 



In fig. 20 (in which the arrangement of the lateral buds 

 does not correspond with that in fig. 19) the zooecia have been 

 numbered in such a way as to facilitate comparison with fig. 

 19. The zocecium "5" is already complete, while "6" is 

 beginning to free itself from the growing-point. The growth 

 of the ovicell has progressed, the most swollen part is already 

 completed, and the aperture (still closed by a membrane of 

 uncalcified ectocyst) is beginning to constrict. 



In fig. 21 the zocecia '^6" and "7" are complete. The 

 aperture of the ovicell is still further constricted, and now 

 consists of a slit-like portion which will soon close completely, 

 and of a wider portion which will become the base of the 

 tubular aperture so characteristic of this species (C. ramosa). 

 A comparison of figs. 19 — 21 with one another will show that 

 the method of the growth of the ovicell has been such as to 

 bring its distal portion on to the front of the branch, while its 

 proximal portion is lateral, and in series with the zooecia. The 

 base of the tubular aperture thus comes to be situated at about 

 the middle line of the internode. 



The valve of the ovicell (17, pi. xii, fig. 10) is formed as a 

 ridge from the back of the ovicell at a stage between figs. 20 

 and 21. The growth of the ovicell will be completed by the 

 outgrowth of the tubular aperture. 



So far as I have been able to make out, the aperture is closed 

 by the uncalcified membrane of ectocyst at all stages of its 

 development, and does not become actually perforated until 

 the escape of the first larva. I am quite unable to say when 

 and how the process of fertilisation is effected. 



