EMBRTONIO FISSION IN OYCLOSTOMATOUS POLTZOA. 25 



a protoplasmic reticulum, which may be richly developed, and 

 which no longer contains any embryos or larvse. 



It must notj however, be assumed that all ovicells in this 

 condition have passed through an embryo-producing stage. I 

 have repeatedly obtained evidence to show that degeneration 

 of the ovicell may start at almost any stage in its development. 

 The external form of the ovicell may develop completely, even 

 if the embryo and its follicle are degenerating. It is common 

 to find an ovicell which, from its proximity to the growing- 

 point, should be a young one, but which appears completely 

 empty in sections. I can only assume in these cases that the 

 protoplasmic structures which the ovicell at first possessed have 

 been absorbed through the pores into the neighbouring zooecia. 

 In other cases the ovicell may contain remains of a degene- 

 rating follicle, the degeneration having clearly commenced 

 before the follicle became vacuolated. One may, therefore, 

 distinguish between a " primary" degeneration of the ovicell 

 occurring before any larvae have been produced, and a " secon- 

 dary " degeneration, which has taken place after the escape of 

 the last larva. It is sometimes possible to distinguish between 

 these two conditions by reason of the fact that the base of the 

 young ovicell is limited by a marked epithelial arrangement 

 of its endocyst (cf. figs. 6 and 9). 



I have so far avoided the use of the term " endoderm" as an 

 equivalent for the inner layer of cells of the secondary em- 

 bryos. This layer is excessively distinct in the embryos soon 

 after their liberation from the embryogenic mass of cells. It 

 then forms an epithelium, lying more or less close to the inner 

 surface of the ectoderm-cells, and completely surrounding the 

 whole internal cavity of the embryo (fig. 33). 



In later stages the distinctness of the cavity becomes lost, 

 and its lining cells send off processes which grow across the 

 cavity and convert it into an irregular set of spaces. At the 

 sides of the sucker these spaces disappear altogether, while 

 between the sucker and the middle of the aboral pole the 

 cavity remains distinct for a time longer. Its cells become, 

 however, almost indistinguishable from the epithelium of the 



