vol. 3.] Robertson. — Embryonic Fission in Crisia. 145 



forms the ocecium is probably true, viz., thai ovary and bud 

 together constitute the equivalent of a sexual animal, the nutri- 

 tive portion of which, the polypide, has undergone a change of 

 function. The complete envelopment of the egg by a polypide 

 bud is similar in the two cases, but in Plumatella this bud is 

 set off structurally at an early stage, whereas in Crisia any bud 

 may be thus set apart, no structural difference between it and an 

 ordinary bud being at first discernible. 



In its main features, the processes of embryonic fission as 

 described by Harmer for Crisia ramosa and other Cyclostomes 

 have been confirmed by this investigation, while certain additional 

 facts and individual variations have been noted. Observations 

 have also been made on the origin of the sexual elements and 

 their secondary union with the polypide buds. The results may 

 be summarized as follows: 



1. — Iu the genus Crisia the sexual elements are produced 

 in both male and female colonies, at the edge of the growing tips 

 of the colony. The germ cells arise from the mesodermal layer, 

 and are differentiated at a point anterior to the budding zone, 

 and at a time earlier than the origin of the buds. 



l^. — In the male colonies of Crisia eburnea a few of the primi- 

 tive germ cells attach themselves to each bud as it arises, and 

 these form the beginning of the testis. In a majority of cases 

 degeneration of the testis probably occurs before the spermatozoa 

 become mature. 



3. — In the female colonies the ovaries are produced at the 

 anterior edge of the young tips. As in the male colonies, in 

 order that the germ cells may reach maturity, it is necessary 

 that they uuite with a polypide bud. In this case one of two 

 results may follow: 



". — The ovum may develop into an embryo, while the polypide 

 bud as such, becomes aborted. 



b. — The polypide bud may develop, while the ovum either 

 degenerates at once or soon after it has passed through 

 the early cleavage stages. 



Many ova are produced which never form a union with a 

 polypide bud. These soon degenerate. 



