350 KOREN AND DANIELSSEN ON THE 



2. Each ovum consists of a chorion and albumen, of a vitel- 

 lary membrane, and of a vitellus composed of larger or smaller 

 globules. Its diameter varies from 0'25 7-0*264 millim. In 

 the egg, when laid, we have never been able to observe either 

 germinal vesicle or spot. 



3. The cleavage which occurs in other mollusks does not 

 take place in these animals. 



4. The ova begin to approximate towards the eighteenth day, 

 and the chorion is detached. The yelk, more or less laid bare, 

 invested only by its very firm membrane, is enveloped by the 

 viscous albumen-like liquid. 



5. Some days later, the ova, even those which were most 

 distant, approximate and form only a single mass, whose dif- 

 ferent portions, larger or smaller, have become grouped ; so that 

 each group, usually composed of from six to sixteen ova, is 

 distinguishable by the naked eye. 



6. On the twenty-third day, these groups are still more 

 distinctly marked out, and are invested by a very delicate mem- 

 brane peculiar to each group, which has by this time taken on 

 an oval or reniform shape : the ova are connected together, and 

 the liquid which enclosed them has lost its viscosity. 



7. Towards the twenty-fifth day the groups possess a more 

 decided membrane and boundaries. Many of the ova which 

 have remained isolated and simple appear as embryos, whilst 

 the others are attached together. 



8. The embryo thus formed consists of a delicate membrane 

 enclosing many ova. 



9. The number of the ova grouped to form one embryo varies 

 greatly, amounting in some cases to as many as a hundred and 

 more. 



10. The number of embryos in different capsules varies ; 

 commonly it is from six to sixteen. 



11. The first organs which are formed after the membrane in 

 question are the rounded lobes with cilia and cirrhi. [The em- 

 bryo then begins to move.] At a later period, the foot, the 

 mantle, the shell, the auditory organs, the proboscis, the eyes, 

 the salivary gland, the heart, and the contractile bladder appear; 

 still later, the digestive and the nervous systems and the 

 branchiae, are developed. 



