No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 27 



7. The Second Somatoblast. 



We have already become acquainted with the lineage of this 

 cell ; in the last section we saw further that its first division 

 was equal and bilateral. It will now be in place to trace the 

 further history of the two resulting cells up to the time of their 

 inclusion within the segmentation cavity as the mesoderm 

 teloblasts. Their position is immediately behind the ento- 

 meres (PI. IV, Fig. 46). In this stage the products of the 

 first somatoblast are beginning to overgrow them. 



The next division of the mesomeres (PI. V, Fig. 60) is 

 peculiar. It is very unequal, and the two small cells m, m, 

 are budded forth just on the posterior lip of the blastopore. 

 This tallies exactly with Nereis. In Nereis these superficial 

 divisions are continued for some time, but in Unio such is not 

 the case. Soon afterwards the mesomeres are included within 

 the segmentation cavity, and take up their definitive position 

 behind the archenteron (PI. V, Figs. 66 and 67). v. Wisting- 

 hausen has observed that in Nereis dumerilii about one half 

 of the second somatoblast remains within the bounds of the 

 ectoderm as the "untere Urzellen des Rumpfes," and forms 

 the anterior part of the ventral plate. 



According to Stauffacher's recent observations (No. 59) on 

 Cyclas cornea, two cells placed symmetrically on either side of 

 the middle line divide in such a way that about one half of 

 each cell comes to lie in the segmentation cavity. According 

 to Stauffacher the two cells in the segmentation cavity are 

 the protoblasts of the mesoderm ; the two cells on the surface 

 the protoblasts of the entoderm. It is to be observed, how- 

 ever, that the last point is a pure assumption. The cells in 

 question cannot be, or, at least, have not been, traced into the 

 entoderm ; and there seems to be a total lack of points of 

 orientation for fixing on the region of these cells, and of the 

 later entoderm cells as identical. I am inclined to think that 

 the surface cells correspond to the cells budded off on the 

 surface by the mesoblast in Nereis and Unio, and that the 

 entomeres lie in front of this point in the form of a plate of 

 relatively small cells. This view, at any rate, accords more 



