X04 John H. Gerodld, 



on the posterior lip of the blastopore) and that in Trochus, Umbrella 

 and other Molluscs (viz. anterior to the teloblast and toward the 

 centre of the egg-). Robert (1903, p. 181) has pointed out that this 

 difference in position may be due to the fact that the telomesoblasts 

 in the former case have not yet sunken in the process of invagi- 

 nation. When this process takes place in Trochns, the mother cells 

 rotate slightly, the small cells being carried inward and forward into 

 an anterior position. 



The fate of these small cells in Phascolosoma is uncertain. They 

 cannot be connected with the formation of the proctodaeum, as 

 Wilson (1898) has shown to be the case in Nereis, for in Phascolo- 

 soma the proctodaeum is to be formed later, at a considerable distance 

 from the point where these cells are found, and in a position that 

 is dorsal rather than posterior and terminal. I have found no trace 

 of these cells, or of descendants of them, in sections of embrj^os 

 twenty four hours old (Fig. 43, 45). At fourteen hours they lie 

 compactly between the coelomesoblast and the endoderm. There is 

 no segmentation cavity, and I cannot affirm that they are more 

 closely joined to one layer than to the other. Corresponding cells in 

 other forms take part in the formation of the wall of the archenteron 

 e. g. in Nereis and Aricia (Wilson, 1898), in PodarJce (Teeadwell, 

 1901), in Tlialassema (Torrey, 1903), and in Crepidula (Conklin, 1897). 

 In certain Annelids and Molluscs, on the other hand, the first 

 derivatives of the telomesoblasts develop as an integral part of the 

 mesoblast bands, viz. in Umbrella (Heymons, 1893), PlanorUs (Holmes, 

 1900), AmpUtrite (Mead, 1897) and Arenicola (Child, 1900). 



The coelomesoblast in an embryo of fourteen hours consists of 

 groups of three or four large cells of nearly equal size (Fig. 40a, 

 41, 42). Ten hours later these cells have multiplied and form inde- 

 pendent lateral bauds (Fig. 43, 45), each with a teloblast still visible 

 at the posterior pole, and already partially diiferentiated in front 

 into somatic and splanchnic layers. 



The endoderm cells at the beginning of this period are flask- 

 shaped, retaining for a long time their connection with the surface 

 at the blastopore by their narrower extremities (Fig. 40a, 40b). A 

 large flat mesial endoderm cell [eii'drm. d) lies dorsally in the sagittal 

 plane, and separates the two groups of coelomesoblast. The stomato- 

 blasts, immediately in front of the blastopore, are now actively 

 dividing. The closure of the blastopore and the formation of the 

 stomodaeum take place between the fourteenth and twenty-fourth 



