of Mollusks in Holothurise. 29 



which seem to take part in the development of the spermatozoa. 

 With these latter the whole capsule is filled ; they are partly- 

 united into moving bundles by their heads, and partly they swim 

 about separately. Their heads are sometimes rounded, some- 

 times elliptical, not uncommonly somewhat pointed anteriorly } 

 the tail is very long, and at times seems to be the prolongation 

 of a sort of ridge upon the head. The end of the tail always 

 presents a longish enlargement. The head is about -^^Qth. of a line 

 in diameter ; the whole spermatozoon ^^^jth of a line or more. 



I have so often (eight times) observed the sperm-capsules, that 

 their occurrence is perfectly certain. They are found for the 

 most part only in those sacs which do not contain free and 

 fecundated ova, but I have also found them in sacs in which the 

 mollusks had commenced their development. 



In these, however, some remains of the ovary are always to be 

 found. 



The spermatozoa may become free by the dissolution of their 

 capsules. I have seen them free in a single case; here they 

 whirled about in great quantities in the ciliated sac round the 

 ovary close to its inner end, that is to say, as far as possible from 

 the place of their development in the neighbourhood of its outer 

 end. 



The development of the mollusks from these yelks proceeds 

 thus: in those sacs in which the contents of the ovary have 

 already passed into the common sac, we find the developing 

 germ mass always inclosed in vesicles of -f^, -^q to y^^ of a line 

 in diameter, which are developed with the sac. Each vesicle will 

 contain fifteen to thirty or more germs, or already developed 

 mollusks. If the germs be not yet developed into mollusks, the 

 characteristic yelk-granules are immediately recognizable in the 

 yelk, together with the germinal vesicle and the finely granular 

 mass with molecular motion of which it was composed in the 

 ovary. Even in the interior of the mollusks, the remnants of the 

 yelk possess the same characteristic yelk-granules. 



I have seen the germ mass within the vesicles in the following 

 states : — 



1. In every vesicle (there may be more than 100 of them in 

 the sac) there are no single yelks or germs, but the yelk-mass is 

 quite difi'used ; many round masses of yelk-granules may indeed 

 exist in it, as in the ovary, but these are much smaller than the 

 pre-existing yelks of the ovary. The germinal vesicles, answer- 

 ing in number to the embryos to be formed in the vesicle (therefore 

 some fifteen to thirty), are scattered through its contents and 

 have still the same constitution and size as in the ovary. Those 

 vesicles in which the yelk-mass is in this finely divided condition 

 always have a white appearance ; the yelk-granules are still as in 



