316 



GILMAN A. DREW. 



This material is probably passed posteriorly by the action of 

 the cilia on the mantle, and very likely the respiratory cur- 

 rents of water swell it into a sort of bubble that remains 

 attached to the posterior ends of the shell-valves, and, while 

 still soft, adheres to the foreign particles with which it comes 

 in contact. 



That the hypobranchial glands are concerned in the forma- 

 tion of the material from which the brood-sac is formed is 

 indicated by their appearance before and after the sacs have 

 been formed. In females in which the ovaries are still full 

 of eggs, the cells of the hypobranchial glands are large and 

 gorged with secretions, while in females that have formed 

 the brood-sacs the cells are shrivelled and almost devoid of 

 secretions. 



The eggs are deposited in the brood-sac (fig. 1), and in it 

 the embryos are carried until they reach an advanced stage 

 in development, probably for a period of three or four weeks. 



The eggs of this species are brown, opaque, few in number, 

 and correspondingly large. From about twenty to seventy 

 may be found in a sac, and they average about '21 mm. in 

 diameter. Each egg is enclosed in a membrane that is pro- 

 bably secreted by the egg, but its formation has not been 

 observed. Fertilisation is probably accomplished in the 

 brood-sac. Eggs and young embryos do not live well after 

 they are removed from the brood-sacs, so the ages of the 

 various stages have not been determined. Processes of 

 maturation and cleavage proceed slowly. The time between 

 the appearance of the first and the second polar body is 

 frequently as much as two hours, and the time between 

 cleavages seems to be nearly or quite as long. It is not 

 beyond doubt, however, that the removal of the eggs from 

 the brood- sacs influenced the length of time. That develop- 

 ment is slow is not to be doubted. Embryos taken from the 

 brood-sacs of specimens kept under as nearly natural condi- 

 tions as possible for a month, had only reached the stage 

 where two gill-lobes were formed. 



It seems probable that the polar bodies may be formed by 



