218 TORREY 



IV. The Mesoblast. 

 I . Coelomesoblast. 



The two coelomesoblast cells (M, M) are the last to sink in 

 (PL II, Fig. 23) at gastrulation instead of the first, as is the case 

 where the development is more direct {Nereis^ Ampliitrite). 

 The right cell is at this time somewhat larger than the left. 

 Immediately after invagination each buds off a small but func- 

 tional cell toward the blastopore (Text- Fig. 8, E\ PI. II, Fig. 

 24). The right one being perceptibly larger than the left, the 

 two pole-cells of the coelomesoblast bands are thus rendered 

 equal in size. Disregarding the latter for the moment, let us 

 follow the fate of the small cells {e, e). Soon after their forma- 

 tion they migrate upwards and away from the blastopore (Text- 

 Figs. 8, F, and 9, A, B, C), become entirely surrounded by the 

 entoblast (Text-Fig. 6, A) and are thus separated from the two 

 coelomesoblast cells. The close association of these small cells with 

 the entoblast (Text-Fig. 6, B and C^ continues during the trans- 

 formation of the latter into the enteron and when last distin- 

 guishable from their neighbors (about 1 8 hours), because of their 

 small size, lighter stain and comparative freedom from yolk 

 granules, they are incorporated in the wall of the intestine very 

 close to the anal region (Text-Fig. 9, D). It seems therefore, 

 beyond all doubt, that they are entoblastic in this case and take 

 part in the formation of the same region of the intestine as do 

 the corresponding cells in Crepidida. 



Similar cells have been described as being budded off from 

 the M's, in at least 16 species of annehds and molluscs and 

 most diverse accounts of their fate have been given by different 

 observers. In four forms they seem to be certainly entoblastic 

 (^Crepidula, Nereis^ Podarke and Thalassema) and probably also 

 in Dreissensia, Patella^ Serpidorbis, Cyclas and Aricia. On the 

 other hand, in at least four forms corresponding (homoblastic) 

 small cells are said to contribute entirely to the mesoblast 

 [Aniphitrite, Arenicola, Umbrella and Plajiorbis) and possibly 

 also in Unio and Physa. Between these two groups, the con- 



