338 PROCEEDINGS OF THE ACADEMY OF [April, 



of 4D have divided, and when there are present about 44 cells (fig. 25), 

 4d or, as it hereafter will be designated more usually, the mesento- 

 blast, ME, di\ddcs dexiotropically into cells of equal size. Before their 

 next cleavage occurs the egg contains about seventy cells (fig. 42). By 

 this division, which is bilateral, one small cell arises anteriorly from 

 each of the large ones (figs. 42, 49). The small cells, E^ and E^ corre- 

 spond to the "Primary Enteroblasts " of Conklin, and will be so desig- 

 nated. Considerable variation may be observed in different eggs as to 

 the later position of these cells, as in some they appear to have moved 

 backward along the sides of the large cells, Me\ Me-, from which they 

 arose, but, as a rule, they remain in close relation to 4D, and always in 

 later stages may be seen associated with the derivatives of this cell, from 

 which it is hard to distinguish them (PI. XXIX, figs. 71 , 73). The large 

 cells soon divide again into almost equal parts, though the posterior 

 and dorsal pair (m^z\ m^z^) are slightly smaller (fig. 71). These latter 

 soon divide again, giving rise to two small cells, z^ and z-, which are 

 posterior to the larger (fig. 73). Just before this cleavage the two 

 cells M^e\ M^e^ divide, giving rise anteriorly and toward 4D to two 

 small cells, e^ and e^ (corresponding to the "Secondary Enteroblasts" 

 of Conklin), which lie close to the first pair of small cells, E^, E^, the 

 four forming a group of little cells with deeply staining nuclei in close 

 contact with 4D, 5C and 5B. Behind them lie the large cells M\ M-. 

 In the nomenclature used these would correspond to "Mesoblastic 

 Teloblasts," but before they begin to function directly as such each 

 again divides, giving off a small cell laterally, and these two cells appear 

 to be dorsally directed toward the cleavage cavity above and to the 

 sides of the enteron, but may remain associated with E\ E-, e^ and e-. 

 However this may be, the mesoblastic teloblasts soon begin to divide, 

 •giving off an irregular row of cells which extend around the gastrula 

 laterally. The cells m^ and m^ also behave in a similar manner, their 

 derivatives being closely associated with those of the large teloblasts. 

 In figures 80, 81 and 82 only the derivatives of the latter are shown, 

 the other lying dorsal to them. As the teloblasts and the cells m^ 

 and m^ di\dde they diverge laterally and leave behind and between 

 them the smaller cells E^, E^, e^, e^, closely associated with the posterior 

 elements of the enteron. When these cells are first given off they 

 lie decidedly above the level of the enteric invagination projecting 

 upward into the cleavage cavity, and while in this position might well 

 be characterized as mesodermal elements ; but later they change their 

 position, slipping in between the teloblasts and the posterior cells of the 

 enteron, and by the time the teloblasts begin to separate and wander 



