344 PROCEEDINGS OF THE ACADEMY OF [April, 



a mesoblastic fate was assigned, are entoblastic in nature, and the same 

 he thinks probably to be true of Aricia and Spio. 



Child (1900) has found for Arenicola that 4d after its first cleavage 

 forms mesoblastic teloblasts, from which later arise two bilaterally- 

 placed mesoblastic bands ; all these cells are mesoblastic in fate, and it 

 is evident from his figures and discussion that he does not find here any 

 entoblastic material. Though in Sternapsis the lineage was not fol- 

 lowed so far as that of Arenicola, Child reaches the same conclusion, 

 and particularly in the latter case he states that the mesoblastic cell 

 is "purely protoplasmic and without yolk". 



In the Annelid Podarke (Treadwell, 1901) 4d arises, together with 

 the other members of the fourth quartet, at the sixty-four-cell stage 

 and is equal in size and appearance to them. It sinks inward with 

 the invagination which forms the enteron, divides and lies in close 

 connection with the endodermal cells. By this di\dsion from the 

 larger cells four small cells are given to the enteron, while the remaining 

 two are purely mesodermal. 



Torrcy (1902), in a preliminary on the cytogeny of Thalassema, 

 assigns to the two small cells arising from the teloblasts the fate of 

 enteroblasts, in a similar manner as in the Annelids above considered. 



Segmentation of the Entohlast. 



Shortly after the origin of the mesentoblast 4d, when the egg contains 

 forty-one blastomeres, all the "macromeres" except 4D are seen to be 

 dividing Iseotropically (fig. 24), with the result that three large cells, 

 4a, 4b, 4c, are given off from their respective macromeres. These 

 cells are slightly greater in size than those centrally grouped, but are 

 not so large as the cell 4d, and on this accoimt we find that of the four 

 cells, 4A, 4B, 4C and 4D, the last is the smallest, nor does it again 

 divide until over one hundred and fifty blastomeres are present. 

 The position of the fourth quartet may be seen in fig. 25 and 

 those following. When the egg contains over eighty blastomeres, 

 4 A, 4B and 4C again divide into equal moieties, the outer three of which 

 (5a, 5b, 5c) lie to the right of the central group. All these cells have 

 become much flattened and form a comparatively thin roof over the 

 segmentation cavity, into which as yet invagination has not begun. 

 The mesentoderm has sunken completely beneath the external layer 

 and extends forward as far as the center of the cavity (figs. 45, 57). 

 At a much later period, when there are nearly one hundred and fifty 

 cells present, 4a, 4b and 4c again divide (figs. 71, 72, 73), giving off 

 small cells to the left and outwardly (4a\, 4b\ 4c^). The invagination 



