ANATOMY AND LIFE HISTORY OF MOLLUSCA. 179 



pouches of the ovary which are lined by flattened germinal 

 epithelium. These cells or rather some of them, enlarge and 

 become ova, remaining attached to the walls of the pouch by stalks. 

 At this stalk there is a perforation of the investing membrane, in 

 some species at least, which becomes micropyle. 



The function of this is to assist in the nutrition of the ovum 

 during its development. The eggs of the Lamellibranchiata are 

 not only remarkable in the possession of a micropyle, but in certain 

 peculiarities of the yolk and of the germinal vesicle. The 

 technicalities of this portion of the subject it is impossible for me 

 to enter upon. There is however one part of the history of 

 development where observations can easily be made. When the 

 ovum has been fertilized, the immediate result is its segmentation 

 or the division of the ovum successively into two, four, eight, &c. 

 successive parts. Where this is equal, it is called regular 

 segmentation. In Mollusca however, unless in rare instances, 

 segmentation goes on unequally. The ovum divides into two and 

 and then four unequal segments in the usual vertical planes. In 

 the third segmentation four small segments are budded off from 

 the large ones ; so that there are four small ones in one plane and 

 four large ones in the other. Without following the process to 

 detail, it may be said that unequal segmentation for the ova is 

 the most widely distributed in the animal kingdom, and for 

 Mollusca except Cephalopoda it is typical. 



Passing on to the larva stage, there is what is called amongst 

 Gastropods the veliger stage, when on the surface of the shell- 

 gland a delicate shell becomes developed. In the Lamellibranchiata 

 the veliger stage presents the larva with a ring of cilia, and in 

 the centre a long nagellum. This stage I have often seen in the 

 case of our common rock oyster (Ostrea mordax, Gould). It is 

 not very difficult to witness the whole development of the oyster. 

 Let a female oyster be taken and some of the milt of the male 

 mixed. It is not long before the ova are developed. By the aid 

 -of warmth and favourable circumstances eggs may be hatched in 

 two hours. About twelve hours after the free-swimming stage 

 the shells begin to appear with the germs of other organs. The 

 shells are distinct from the first, and for a long time have a 

 smooth rounded outline. At the end of six clays the shells nearly 

 •cover the embryo, which is hardly more than visible to the 

 unassisted eye as the merest grain. All the larger organs can be 

 seen with a microscope, as they are then separated, with the 

 exception of course of the reproductive tissues such as ovary and 

 milt. After swimming about in this stage for a few days, the 

 period of w T hich has not been exactly ascertained, the young 

 oyster, the so-called " spat," fastens itself by one valve of its shell 

 i;o any rough and clean body. It is then very thin and delicate, 



