28 Prof. Miiller on the Development 



into small round masses, and among them are found many ex- 

 cessively minute particles which exhibit the molecular motion. 

 The yelk-granules are for the most part naked; a few of the 

 larger ones are however surrounded by an albuminous area 

 such as has been remarked in the ova of sharks and frogs. The 

 ova cannot be separated from one another ; on pressure, their 

 membranous investment bursts, and some become no longer round, 

 but elongated or pyriform ; it would appear, that the membrane 

 investing the yelks belongs rather to the ovarian capsule, which 

 it serves to divide into compartments, and that the ovum has no 

 proper vitellary membrane. The ovum which has left the 

 ovarium has certainly no vitellary membrane, in this respect 

 resembling the ovum of Actceon as described by Vogt. When the 

 ovary is perfectly developed, it and its capsule dehisce, and give 

 exit to the ova which thus are found within the molluskigerous 

 sac. Here fifteen to thirty together become invested by a 

 common vesicle, and these vesicles occupy partly that portion of 

 the sac which follows the ovary, partly the space between the 

 capsule of the ovary and the sac. 



Under these circumstances the development of the mollusks 

 commences, and is first recognizable by the occurrence of the 

 phenomena of yelk-division. For a long time, with so much else 

 that was incomprehensible, I was unable especially to comprehend 

 how it was that the yelks began at once to develope their em- 

 bryos, since as a general rule the division-process arises as a 

 consequence of fecundation, and is never observed in gemmae. 



Towards the beginning of September, however, I discovered 

 the fecundating organs of the mollusk-ova in the same sac which 

 contains the ovarium with its capsule. 



There are generally many sperm-capsules present ; in most 

 cases there are four to five or eight, and there may be as many 

 as eighteen. They lie perfectly free in a rather wide portion 

 of the molluskigerous sac, not far from the ovary and somewhat 

 nearer to its aperture. 



The sperm-capsules are elliptical bodies ^th to f ths of a line 

 in diameter, which are not ciliated upon their surface. Each 

 consists of two membranous sacs, one inclosed within the other ; 

 the outer is the larger, and projects beyond the inner anteriorly 

 and posteriorly. An epithelial layer lines the simply mem- 

 branous outer sac, and between it and the inner sac there are 

 clear cell-like globules of different sizes, together with solitary 

 and aggregated yellow fat-granules, like those of the ovarian 

 capsule. The internal sac is a perfectly transparent, simple, 

 structureless membrane, though I have repeatedly observed 

 sudden contractions of its walls under the microscope. Its inner 

 surface is clothed with a layer of cells g^th of a line in diameter, 



