356 Messrs. Koren and Danielssen on the 



which cause the eggs to press against each other still more 

 (lig. 1). We cannot as yet find any trace of the formation of 

 organs ; the exuded mass is homogeneous and semitransparent, 

 but usually becomes more and more abundant above, until at 

 last it exhibits on the upper part of the membrane some ex- 

 tremely fine cilia. Soon after the formation of these, we see some 

 cirrhi, and it is only then that the movements of the embryo 

 commence. As the cirrhi become more numerous and larger, the- 

 movements become more and more lively ; and shortly afterwards 

 the embryo detaches itself from the common mass. This separa- 

 tion sometimes takes several hours, and often carries away with it 

 a portion of the eggs, which, being still separated from the em- 

 bryo, die. The above-mentioned exudation, to which we have 

 paid attention in our first memoir, differs in its quantity ; for 

 sometimes it happens that it only makes its appearance as a 

 border or clear line within the membrane, and in other cases it 

 may be very considerable before the embryo has detached itself. 

 The number of eggs which take part in the conglomeration to 

 form the embryo varies as much as the quantity of embryos in 

 the different capsules. The ordinary number of eggs which 

 collect to form one embryo is from thirty to sixty, but we have 

 very frequently found that it was composed of 130 eggs. It is 

 to be observed, that the fewer the individuals [embryos] in a 

 capsule, the greater the number of eggs of which they consist, 

 and consequently the individuals are always larger. The form 

 of the embryos also undergoes some variations, but they are 

 ordinarily oval or reniform. The number of individuals also 

 varies greatly; thus sometimes we have found five or six, some- 

 times eighteen to twenty-four, and even thirty-six. Gray says 

 that a capsule contains more than 100 eggs, and that only four or 

 five young are excluded from it. Howse shows us twenty-four; 

 he also describes egg-capsules, which, he says, belong to Fusus 

 Norwegicus and Turtoni, and of which those of the former only 

 contain two or three embryos, and those of the latter six. 



Now that we have seen how the eggs are grouped together, 

 and how the exudation takes place, as well as how the com- 

 mencement of the embryo is formed, we may follow their deve- 

 lopment. When the cilia and the cirrhi make their appearance 

 in sufficient quantity upon the upper part of the membrane, the 

 differentiation of the organs commences in this exuded and 

 homogeneous mass, for we begin to see slight outlines of the 

 rotatory organs, which then very rapidly acquire their peculiar 

 forms. But almost at the moment of their appearance we observe 

 the foot as a dense and almost square mass, which is still ad- 

 herent to the rotatory organs. As soon as these increase in size, 

 the foot acquires a more and more rounded form, detaches itself 



