No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 39 



generation of ectomeres is composed of very large cells. 

 Why ? Because they form early and voluminous organs (larval 

 mantle). The left member of this generation is larger than 

 the right. Why } Because it contains the larval mesoblast. 

 The entomeres are very minute. Why, again } Because the 

 intestine remains rudimentary until a late stage ; thus a par- 

 allel instance to the apical-pole cells. One can thus go over 

 every detail of the cleavage, and knowing the fate of the cells, 

 can explain all the irregularities and peculiarities exhibited. 



These peculiarities of cleavage are all due to the precocious 

 segregation of organs or tissues in separate blastomeres. The 

 order and character of the segregation again are ruled by the 

 needs of the embryo. Thus, one of its greatest needs is the 

 large and powerful shell with which it is provided. The 

 necessity of such provision being made has caused the produc- 

 tion of a large shell-gland, which has impressed itself on the 

 segmentation stages as the largest of their blastomeres. I 

 could illustrate the principle in each of the cases just enumer- 

 ated, but will be satisfied with repeating the introductory 

 sentence of this paragraph in a more special form : The pecu- 

 liarities of the cleavage in Unio are but a reflection of the 

 structure of the glochidium, the organization of which controls 

 and moulds the nascent material. 



The larval mesoblast of Unio must be regarded as a kind of 

 massed mesenchyme, which has relations to the primary body 

 cavity only. I consider it extremely probable that in other 

 lamellibranchs, and in a wide series of forms besides, ecto- 

 dermal cells pass into the primary body cavity and function as 

 mesenchyme, producing muscle-cells, or, as in the case of the 

 head kidney of Nereis, organs of excretion, or, again, support- 

 ing-tissue. Goronowitsch (No. 44^) and Miss Piatt (No. 56a) 

 have both recently certified to the derivation of the bases of 

 certain tissues of the head of vertebrates, hitherto considered 

 mesenchymal, from the epiblast. It may seem fanciful to 

 collate such scattered observations from so widely different 

 classes of animals, but I believe not. On the contrary, it 

 seems to me that the very separateness of these observations 

 gives them a significance all the more apparent. It will help 



