XC11 INTRODUCTION. 



cell. It is usually, but not always, terminal, and is pro- 

 duced by gemmation from the wall of the zooecium ; its 

 interior is in direct communication with the perigastric 

 cavity. In certain species (e. g. Bicellaria ciliata), the 

 ooscium exhibits a somewhat complex structure. The 

 opening is closed by a membranous capsule (attached by 

 its base in front), which partially occupies the cavity. 

 This capsule is furnished with muscles, by which it can 

 be more or less withdrawn from before the orifice, so as 

 to allow of the escape of the larva. The ovum, when 

 transferred to the ovicell, lies between its outer wall and 

 the capsule. By the action of the retractor muscles, the 

 membranous wall of the capsule can be drawn down, so 

 as to leave a larger space for the embryo as it increases in 

 size, as well as secure a passage for it when mature *. 



At the time when the reproductive elements are pre- 

 sent in the cells, ova are commonly found in the ooecia, 

 where they occur in every stage, from the first segmenta- 

 tion to the perfect larval condition. Huxley was the first 

 to suggest f that the ovicell must be regarded as a kind 

 of marsupial chamber, into which the ova migrate from 

 the zooecium for the purpose of completing their develop- 

 ment ; and this view has met with general acceptance. I 

 was at one time led, by observations which I had made on 

 a number of species, to challenge it, and to contend that 

 the ovum found in the ovicell was produced there, and 

 was not a mere emigrant; but, as I have already ex- 

 plained J, I am now convinced that the marsupial theory 



* The structure and development of the ovicell have been admirably de- 

 monstrated by Nitsche (Zeitsch. &c. xi. 1. Heft, pi. i. figs. 10, 11). 



t Quart. Journ. Micr. Sc. iv. (1856), p. 191. 



J " Contributions to the History of the Polyzoa," Quart. Journ. Micr. Sc. 

 (n. s.) xiii. p. 30. 



