82 Royal Society : — 



General Considerations relative to the Observations contained in 

 Sections II., III., IV. (containin;/ the developmental histories of 

 Pisidium, Aplysia, Tergipes, Polycera, and Xeritiua). 



In these observations the author points out briefly their bearing 

 on two matters of theoretical importance, \az. (1) the origin and 

 significance of what has been called the Gastrula-^\iViiie of deA'elop- 

 ment, and (2) the homologies or homogenies (as the author prefers 

 to say) of the shells, ligaments, and internal pens of the Mollusca. 

 More facts have to be sought out and brought to bear on these 

 questions ; but the author, while occupied in that further search, 

 indicates the anticipations which must guide and stimulate it. 

 Before doing so he mentions that there are a variety of other 

 matters of interest in the facts recorded in the paper which cannot 

 yet be brought into any theoretical structure, but which are not 

 on that account kept back, as they A\ill probably be of some service 

 in their isolated condition. 



Kowalevsky was the first to describe, in a precise manner, the 

 formation of the foundations of the alimentary tract in a develop- 

 ing embrvo, by invagination of the wall of a simple primitive 

 blastosphere, or hollow ball of embryonic cleavage-corpuscles. He 

 detected this mode of development in AnijMoxus, and subsequently 

 in Ascidia. By later researches he was able to indicate the same 

 mode of development in certain Vermes (Sagitta, Eiuuves, Lumhri- 

 cus) ; and he mentioned incidentally that he had observed a similar 

 development in the lieteropodous mollusk Atalanta. At that 

 time the author was studying the development of Pisidium and 

 Limax, and obtained evidence of the invagination of the primitive 

 blastosphere in those two widely separated mollusks. Subse- 

 quently at Xaples he found the same process occurring in Nudi- 

 branchs. The probable identity of this process of invagination 

 with that so well known in the Batrachians, especially through 

 Strieker's admirable work on the subject, became clear, to those 

 occupied with embryologieal studies, fi'om the facts established by 

 Kowalevsky ; and the " anus of Eusconi " could now be recognized 

 in the " orifice of invagination " present in members of the three 

 large groups of Vermes, Mollusca, and Vertebrata. 



The embryonic form produced by this inAagination-process is a 

 simple sac composed of an ectoderm and eudoderm, with an orifice 

 connecting the exterior -with the cavity lined by the endodeinn. 

 It, in short, presents the typical structure of the simplest Coelen- 

 terata, and corresponds exactly with the so-called Planida of the 

 polyps and corals. Hence we are tempted to see in this primitive 

 invagination-form the representative of the Ca^lenterate phase of 

 development of the whole animal kingdom. In a paper published 

 in May 1873*, containing the substance of lectures delivered in 

 the preceding October, the author discussed this notion at some 

 length, and other points connected with the attempt to work out 

 the correspondences of the embryonal ceU-layers of the various 



* Annals and Mag. Nat Hist. 1873, si. p o2 



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