412 HOLMES. [Vol. XVI. 



and in Planorbis, according to my own observations, has yet a 

 different origin, arising from the cells of the third quartette in 

 the two anterior quadrants. It arises, as in Crepidula, at a late 

 period of cleavage, and its origin is likewise bilateral. As the 

 cells of the third quartette are arranged symmetrically on either 

 side of the median axis of the egg, its origin from three quad- 

 rants could not be bilaterally symmetrical. It is quite certain 

 that, in Planorbis at least, no secondary mesoblast arises from 

 the posterior quadrants, unless at a very much later period of 

 development. 



Several cases have been pointed out by Lillie, among accounts 

 of the embryology of the lamellibranchs, where the figures of the 

 authors show strong evidence of the existence of secondary meso- 

 blast. The figures of Cyclas by Ziegler and Stauffacher, of 

 Teredo by Hatschek, of Anodonta by Goette and by Schierholz, 

 of Ostrea by Horst, show mesoblast cells in the early stages of 

 gastrulation that could scarcely have arisen from the pole cells. 

 In all these genera, cells are figured in front of, as well as 

 behind, the blastopore. There are also similar cases in papers 

 on other groups of mollusks. Kowalevsky's figure of a sagittal 

 section of the larva of Dentalium shows a large mesoblast cell 

 in the blastocoel at either end of the gastrula. And similar 

 indications of secondary mesoblast are shown in Fol's figure of 

 a sagittal section of the larva of Firoloides. 



The case of Paliidina vivipera is an interesting one in this 

 connection. It is one of the few points of agreement, among 

 those who have worked on the form, that mesoblastic pole cells 

 do not occur. Tonniges finds that, in this form, mesoblast is 

 produced from certain cells lying in front of the blastopore. If 

 we accept Tonniges's account, the formation of the mesoblast 

 in Paludina would seem to correspond to the formation of the 

 secondary mesoblast in other forms. 



The researches of Eisig and Wilson on the development of 

 annelids suggest that the occurrence of secondary mesoblast 

 may be typical for both annelids and mollusks, and indicate 

 another striking point of agreement to the many which exist 

 between the methods of cleavage of these groups. At the same 

 time they serve to connect more closely the cleavage of anne- 



