106 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



the sixth cleavage, the two primary mesoblast cells (d 6 - 3 , c? 6 - 4 ) are 

 crowded into the yolk beneath the blastoderm, pushing the two en- 

 toblast nuclei deeper into the yolk (Plate 7, Fig. 59). The primary 

 mesoblast cells thus come to lie beneath the blastoderm at the posterior 

 end of the embryo. As in the two preceding stages, they are easily 

 identified by their distinguishing features, and furthermore the divisions 

 of all surrounding cells are accounted for, so that there can be no doubt 

 of the lineage of the primary mesoblast cells. In series of eggs in 

 various phases of the sixth cleavage the primary mesoblast cells have 

 been seen in their successive positions, from that of the thirty-two-cell 

 stage to that of the sixty-two-cell stage. At a time when some ecto- 

 blastic cells are undivided and the blastoderm is not completed, the 

 two primary mesoblast cells are seen filling the blastopore and in part 

 exposed to the exterior, but as the blastopore becomes closed they sink 

 into the yolk, and the blastoderm closes over them. 



The primary mesoblast cells (d 6 - 3 , rf 6 ' 4 ), before the sixth cleavage 

 takes place in them, may be symmetrically placed with reference to 

 the sagittal plane (Plate 7, Fig. 64 ; Plate 8, Fig. 72 ; Plate 12, 

 Fig. 120) ; but more often one (c? 6 - 8 ) is found in a position dorsal 

 or anterior to the other (Figs. 56, 59, 60, 71). In the majority of 

 eggs the two cells appear to have undergone torsion as the blasto- 

 derm closed around and over them. In the thirty-two-cell stage 

 they are usually symmetrically placed side by side, but even in 

 this stage there may be some shifting, as shown in Figure 52 (Plate 

 6). Figures 62 and 63 (Plate 7) show a very common condition, 

 in which they have been so turned that the cleavage plane between 

 them no longer coincides with the sagittal plane. In all such cases 

 they appear to retain their original positions with reference to the 

 right and left sides of the embryo. The various positions occupied 

 by these cells may be the result of shiftings in adjustment to least 

 resistance at the time when the overgrowing blastoderm crowds them 

 inwards. 



The spindles concerned with the sixth cleavage of the two derivatives 

 (c?*' 8 , d 6 - 4 ) of the primary mesoblast cell are more often about perpen- 

 dicular to the long axis of the egg (Plate 7, Fig. 56), but sometimes 

 almost parallel to that axis ; all intermediate conditions are seen. In 

 Figures 65 (Plate 7) and 121 (Plate 12) the two cells are represented 

 as having completed the sixth cleavage, so that there exists a stage 

 with sixty-two cells. Immediately after division the four resulting 

 cells (d 7 - 6 - 8 ) are rounded, as shown in Figure 65 ; but soon afterwards 



