EMBRYONIC FISSION IN OYCLOSTOMATOUS POLYZOA. 37 



bryonic fission in Cyclostomes may have been connected with 

 the presence of the nutritive conditions which are suited to 

 induce the precocious formation of buds. Nothing can be 

 more striking than the obvious continuity of protoplasm 

 between the several units of the colonv in a decalcified branch 

 ofCrisia. In the individuals which are modified as ovicells 

 the protoplasmic network is particularly well developed. The 

 embryo is thus surrounded by a rich nutritive material ; and 

 just as the presence of a nutritive placenta in a Placental 

 Mammal has resulted in the diminution of the size of the 

 ovum, and in various abnormalities in its early segmentation, 

 so in Crisia the size of the egg is reduced to a minimum, the 

 whole of the nutritive substance being retained in the parental 

 tissues and handed on to the egg or embryos as required, while 

 the segmentation is entirely abnormal. Further, while the 

 Mammalian embryo becomes easily comparable with that of 

 any other Vertebrate embryo after a certain number of the 

 early stages have been passed through, so the Crisia larva 

 becomes, to some extent at least, comparable with the free 

 larva of any other Polyzoon, although with this difference from 

 other Polyzoa, viz. that the primary embryo has given rise to 

 numerous larvae, a process comparable with the artificial pro- 

 duction of a complete embryo from a single blastomere of the 

 two cell stage in the experiments of Driesch (9) and of Fied- 

 ler (10). 



Attention has already been called to the similarity between 

 the early stages of the development in Crisia and those in 

 Salpa. The latter is another example of the modification of 

 the first processes in the development, associated with the 

 presence of special maternal nutritive arrangements. The 

 embryo of Salpa develops, as is well known, in close connec- 

 tion with a kind of placenta; and its early stages are, compared 

 with those of most other animals, highly abnormal. The 

 formation of buds from the individual developed from the egg 

 does not take place at once, as in Crisia, but is deferred until 

 the animal is mature, when buds are produced in very large 

 numbers from the stolon. 



