58 CONKLIN— EMBRYOLOGY OF A BRACHIOPOD. [April 4, 



a dense mass of mesoderm cells, except on the dorsal side, where 

 it is still in contact with the ectoderm (Figs. 56-64). At this 

 stage therefore there can be no mesenteries since there is practically 

 no coelom. 



In early stages of larval life the mesoderm cells are mesenchy- 

 matous in the anterior region of the body and epithelial in the 

 posterior regions {^cf. Figs. 43-46). In the later stages the meso- 

 derm cells of the posterior regions become more mesenchyme-like 

 (Figs. 48-52 and 53-55), while in still later stages they become 

 densely packed and pigmented and it is impossible to distinguish 

 their cell boundaries (Figs. 56-64). 



VI. General Considerations and Conclusions. 



Although I am not one of those who expect to find phylogeny 

 '' writ large " in ontogeny, yet it may be worth while to point out 

 the bearings of the development of Terebratulina on the supposed 

 relationships of brachiopods. Since my own studies cover only 

 the embryonic and larval periods, I shall of course limit to those 

 periods the comparison of Terebratulina with other forms. Within 

 these periods we may compare (i) the cleavage, (2) mesoderm and 

 coelom formation, (3) orientation of embryo and larva, (4) the gen- 

 eral morphology of the larva. 



I. The Cleavage. — As has been said already, there is no evidence 

 that the cleavage of Terebratulina resembles that of mollusks or 

 annelids. It is now known that in a great many annelids and 

 leeches and in all groups of mollusks except the cephalopods the 

 cleavage is of a certain determinate or morphogenetic (Child, 

 1900) type. The principal characteristics of this type of cleavage 

 are that the ectoderm is segregated in three quartettes of cells, that 

 the greater part of the mesoderm appears in one cell (4d) of the 

 fourth quartette, and that the remaining cells of the fourth quartette 

 together with the basal cells (macromeres) constitute the endoderm, 

 and finally that the elongation of the embryo takes place by the 

 teleoblastic cleavage of certain cells in the ectoderm and mesoderm 

 (first and second somatoblasts, 2d and 4d) and possibly also in the 

 endoderm (endodermic derivations of 4d). In addition to these 

 general characteristics of the cleavage of annelids and mollusks 

 there are other characteristics less general in application, such as 

 the derivation of the prototroch, the stomodaeum, the cerebral 



