724 PROCEEDINGS OF THE ACADEMY OF [Oct., 



forward, by contact with the cells of the X group. In Thalassenia there 

 occurs a similar shifting apart of the mesomeres, which Torrey (1903) 

 also explains as being caused by pressure of the entomeres. 



The position occupied by the mesomeres is at first sight somewhat 

 different from that occupied by similar cells among the mollusks and 

 annelids. In the majority of these forms the mesomeres, soon after 

 their formation, are invaginated into the cleavage cavity. In Dino- 

 philus they remain on the exterior until covered by ectoderm, when 

 they move laterally to the entoderm. There was, however, at an 

 early stage a cleavage cavity between the ectoderm and the entoderm, 

 and had the mesomeres moved into it, then their behavior would have 

 been that of the corresponding cells in most mollusks and annelids. 

 As it is, their migration into the cleavage cavity is postponed until a 

 later period of the history of the embryo, but their final position is 

 not essentially different from that of other forms. 



In fig. 57 a band of mesoblast cells is seen on each side of the ento- 

 derm, running forward from the mesomeres, which are in division. 

 The mesoblast is also shown in fig. 59 ventral to the entoderm. 



The later history of the mesoblast has not been followed out in detail. 

 It is my hope to be able at a later period to determine precisely to 

 what organs and tissues the mesomeres contribute, but it seems fairly 

 certain that they give rise to the mesenchyme and sex organs (ovaries) 

 of the adult. 



Of especial interest would be the fate of the first two products of 

 each mesomere, in the light of the discovery made by Conklin (1897) 

 that part of 4d is in Crepidula entodermal. 



Following Conklin's discovery, Wilson (1897) in Xereis, Treadwell 

 (1901) in Podarke and Torrey (1903) in Tlialassema found that part 

 of the mesomeres in these forms was entodermal, I have not been 

 able to follow out the fate of the first two products of each meso- 

 mere, since their small size and position causes them to become inextri- 

 cably blended with the ectodermal cells surrounding the posterior lip 

 of the blastopore. The position of three of these cells in the mid-line 

 suggests, however, that their fate may be different from that of the 

 other products of the mesomeres. 



IX.— Axial Relations. 



The discussion of the axial relations of the Dinophilus embrj'o falls 

 naturally under two heads, viz. : (1) The relation of the first and second 

 cleavage planes to the future median plane of the adult, and (2) the 

 shifting of areas which occurs in relation to the closure of the blasto- 



