DEVELOPMENT OF THE PECTINIBRANCHIATA. 333 



membrane, the formation of the embryo begins ; a very clear, 

 finely granular and viscous liquid is deposited at first on the 

 external surface of the ovum. In this plastic mass we see, in 

 some localities, cells developed, in others muscular tubules, ac- 

 cording to the nature of the organ which is to be produced ; 

 thus, the first signs of activity visible about the ova, after they 

 are laid, are observed in the very viscous albuminous mucus 

 which surrounds them, and this activity is manifested by the 

 mere union of a certain number of ova into more intimate rela- 

 tions. We imagined at first, that these phaenomena of agglo- 

 meration might represent yelk-division ; but we soon gave up 

 this idea, which seems to us altogether incorrect*. Besides, the 

 development o^ Purpura lapillus, which we have subsequently 

 examined, and in which division and the phaenomena of agglo- 

 meration go on at the same time, has also contributed to our 

 rejection of this notion. We come therefore to the conclu- 

 sion, that yelk-division is not always indispensable for the 

 production of the embryo ; but the fact of the agglomeration of 

 some fifty or more perfectly formed ova, into a single individual, 

 is certainly strange enough. Where does the formative principle 

 exist? Is it enclosed in an isolated ovum? Or is it extended 

 over the whole mass ? and is it the common power which be- 

 comes then the organizing force of the substance ? We have 

 seen the isolated ova undergoing a certain amount of develop- 

 ment, but the being which results from them is very incomplete 

 and becomes very rapidly destroyed. It would seem to want 

 the materials requisite for its permanent existence. We shall 

 return to this subject in treating of the development of Purpura 

 lapillus, and we shall confine ourselves here to adding, that this 



♦ In publishing here the observations of MM. Koren and Danielssen, which 

 are highly interesting in themselves, apart from the interpretation which may 

 be put upon them, we consider ourselves bound to remark, that the rounded 

 masses of vitelline matter, regarded by these authors to be simple ova, seem to 

 us to be merely vitelline spheres, whose utriculiform envelope presents a little 

 more consistence than ordinarily, and that therefore the aggregation from whence 

 the body of the embryo arises, is the result of the grouping together of the 

 vitelline spheres of a single ovum, and not the product of the union of many 

 primitively distinct ova. What these authors remark with respect to the phaeno- 

 mena of yelk-division, which are also sometimes observed in these spheres, 

 would in nowise oppose this interpretation.— (iVo/? by M. Milne-Edwards.) 



