280 HiNCKS, ON THE POLYZOAN OVICELLS. 



lining membrane, stretching from side to side (fig. 1) . At this 

 stage the ovum has not attained any very definite form. It is 

 simply a mass occupying the space betvreen theendocyst and the 

 Tvall of the ovarium. The first change which I have noticed 

 seems to consist in a slight concentration of the matter at the 

 centre of the nascent ovum. Gradually it assumes a circular 

 form, and segmentation takes place, the mass being divided 

 into four and afterwards into more numerous granules 

 (figs. 2, 3) . I have not detected a germinal vesicle. On 

 the disappearance of the segmentation the ovum exhibits 

 a marginal band of large and somewhat oblong cells, sur- 

 rounding a central, opaque, granular mass, and changes its 

 circular for a more or less oval figure (fig. 4). As the growth of 

 the ovum proceeds the membranous partition which encloses it 

 is pushed downwards, and the sac at last occupies a consider- 

 able portion of the ovicell, suspended, as it were, from the top, 

 and reaching towards the aperture. Its wall is also thick- 

 ened, and shows very distinctly. Indeed, from its first differ- 

 entiation it may be detected without difficulty. 



Subsequently the ovum increases in size until it nearly 

 fills the cavity of the capsule, and 'the containing sac would 

 seem to be ruptured and to disappear. Cilia are at last de- 

 veloped on the surface, and the embryo moves restlessly about 

 the interior of the ovicell, and at last makes its escape through 

 the aperture."^ 



I have never seen spermatozoa within the ovicell, and am 

 unable to throw any light on the way in which impregnation 

 of the capsular ova takes place. 



Dr. Reid mentions having witnessed the division of an 

 embryo into two portions, one of which immediately escaped 

 from the capsule, the other remaining in it for the time, but 

 nothing of the kind has occurred to me. 



A word now as to the ova, which are produced within the 

 cells, and which Professor Huxley supposed to make their 

 way into the ovicell, for the purpose of accomplishing the 

 later stages of their development. 



They are commonly present in cells bearing capsules from 

 which the embryos are being discharged. Professor Huxley 

 has described them as they appear in Bugula avicularia, and has 

 pointed out the respective positions of the ovary and testicle. 

 They present one very distinctive character. They are never 

 ciliated. No observer, I believe, has professed to detect cilia 

 upon them at any stage of their development. Van Beneden 



* I do uot offer tlie foregoing as a complete account of the development 

 of the ovum, but only as an enumeration of certain successive stages of it, 

 which have come under my notice. 



