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 division from the 
larger cells four small cells are given to the enteron, while the remaining 
two are purely mesodermal. 
Torrey (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 Entoblast. 
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 lexotropically (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 account 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, 
4A, 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 
