EGGS AND OVICELLS 5<D7 



The fate of the ripe egg differs widely in different eases. In 

 the Entoprocta it develops in a kind of brood-pouch formed from 

 part of the vestibule. The fact that in Pedicellina (Fig. 243) the 

 embryos grow largely during their development, shows that 

 nutritive material must be supplied to them from the parent. 

 There is reason to believe that the epithelium of the brood-pouch 

 is responsible for this process. The eggs are also known to 

 develop at the expense of nutritive substances prepared by the 

 parent in the ovicells of the Cyclostomata. In other cases, as in 

 some species of Alcyonidium, the egg is large, and its copious 

 yolk doubtless supplies a large part of the material required for 

 development. 



In the Ectoprocta, development takes place in a variety of 

 places. In most Cheilostomata a single egg passes into the 

 globular ovicell, which is formed above the orifice of many of 

 the zooecia. In certain Ctenostomata, 1 Phylactolaemata, 2 and 

 Cyclostomata, 3 the ripe egg is taken up by a rudimentary poly- 

 pide-bud, which is specially formed for the purpose. In the 

 Ctenostomata and in the fresh -water Polyzoa these buds, if 

 present, are found in ordinary zooecia which do not become 

 modified externally in any special way. In the Cyclostomata 

 (Crisia), on the contrary, the formation of the polypide-bud 

 is intimately bound up with the development of the ovicell. 

 The number of the zooecia which produce eggs that are capable 

 of development is greatly restricted in this group. The ovicell, 

 which contains numerous embryos, is not merely a portion of 

 a zooecium, as in the Cheilostomata ; but it is probably to be 

 regarded as a modification of the entire fertile zooecium or 

 zooecia. These take on an appearance widely differing from that 

 of the ordinary zooecia, and in course of time give rise to the 

 ovicells (see Fig. 237). 



In all these cases the egg develops inside the parent, and it 

 was hardly known, before the publication, of the interesting 

 researches of M. Prouho, 4 that some of the Polyzoa lay eggs 

 which develop externally. In these cases a considerable number 

 of eggs are produced simultaneously by a single zooecium. 



1 Joliet, Arch. Zool. Exp. vi. 1877, p. 262. 



3 Kraepelin, Abh. Ver. Hamburg, xii. 1893, No. 2, p. 22. 



3 Harmer, Quart. J. Micr. Sri. xxxiv. 1893, p. 211. 



4 Arch. Zool. Exp. 2 ser. x. 1892, p. 557. 



