720 PROCEEDINGS OF THE ACADEMY OF [Oct., 



blastopore closes, and arc concerned in the formation of the stomo- 

 dseum. Mead (1897) states that in Amphitrite 22?, 2\? and 2c^ come 

 to occupy positions similar to the "stomatoblasts" of Nereis, but is 

 in doubt as to their precise fate. In Capitella, Eisig (1898) finds that 

 the margin of the blastopore is composed entirely of the products of the 

 second quartette ("oesophagoblasts"). In Podarke (Treadwell, 1901) 

 at least one cell from the second quartette forms a portion of the stomo- 

 daeal wall, as do also products of 3a, 3b and 3c. In Arenicola (Child, 

 1900) eight products from the third quartette function as stomato- 

 blasts and form an arc of cells similar to that formed by the stomato- 

 blasts of Nereis. 



Various conditions are found among the ]\Iollusca. In Ischnochiton 

 (Heath, 1899) products of both the second and the third quartettes 

 take part in the formation of the blastopore lips; this appears to be 

 also the case in Planorhis (Holmes, 1901), while in Trochus (Robert, 

 1903) the lips are formed at first by cells derived from both the second 

 and third ciuartettes, but later the products of the second quartette 

 are excluded from the rim of the blastopore. 



In conclusion, it may be said that in Dinophilus no one set of cells 

 3an be denominated stomatoblasts, but that products from both the 

 second and third quartettes take part in the formation of the blasto- 

 pore rim. The products of 2b^-^, 3a and 3b, since they lie immediately 

 in front of the blastopore, probably contribute to the formation of the 

 stomodaeum. The fate of the cells 2a--^-^, 2a--^-^, 2c^-^-^ and 2c^-^- is 

 problematical. It is possible that they, too, contribute to the forma- 

 tion of the stomodseal wall. On the other hand, it seems pretty certain 

 that but a small part of 3c and 3d contribute to the formation of the 

 stomodaeum. Their position, just anterior to the cells of the X group, 

 which is not materially altered during the shifting of the ectoderm, 

 together with their large size, makes it possible that they contribute 

 largely to the lateral ectoderm of the head. 



Mesoblast of ectodermal origin (larval mesoblast, Lillie ; psedomcso- 

 blast, Eisig) has been described for a number of Mollusca, and the 

 annelids Podarke (Treadwell, 1901), Capitella (Eisig, 1898), Aricia 

 (Wilson, 1898) and Thalassema (Torrey, 1903). Schimkewitsch (1895) 

 describes cells from the ectoderm in the anterior part of the Dinophilus 

 embryo as migrating into the body cavity and there contributing to 

 the mesenchyme. Certain cells, of whose exact lineage I am ignorant, 

 belonging to the first quartette, do invaginate (figs. 57 and 58), and it 

 is quite possible that one or more of them may give rise to the mesen- 

 chyme of the head and the mouth segment region; but it is my belief 



