EMBRYOLOGICAL INVESTIGATIONS 259 



animals above Protozoa commence their existence 

 as eggs i.e., as single cells from which all parts of 

 the adult body are ultimately derived. The pur- 

 pose of this arrangement appears to be to secure 

 the advantage to be derived from cross fertilisation, 

 a process which concerns two cells only, male and 

 female respectively ; and which necessitates a uni- 

 cellular stage as the initial one in individual de- 

 velopment. The eggs of Metazoa, while still in the 

 ovary, are very commonly enclosed in follicles. 

 These consist of one or more layers of cells which 

 are of epithelial origin, and which are of the same 

 order as the ova themselves, and in their earlier 

 stages indistinguishable from these. The follicle 

 cells play an important part in the nourishment of 

 the egg which they surround ; but they are left 

 behind in the ovary, or are rubbed off after the egg 

 has ripened and discharged from the ovary, and 

 they have nothing to do with the formation of the 

 embryo. 



In the genus Pyrosoma however, a well-known 

 colonial and pelagic Ascidian, Salensky discovered 

 a very peculiar condition of things. Each Pyrosoma 

 individual produces one very large egg, which is 

 closely invested by a follicle or capsule. The egg 

 is meroblastic ; and as in the hen's egg, segmenta- 

 tion is confined to one pole, and results in the 

 formation of a small cap of cells or blastoderm^ 

 lying on the top of the mass of unsegmented yolk 

 formed by the rest of the egg. As segmentation 

 proceeds, certain of the follicle cells investing the 

 egg grow in between the segmentation cells or 



