108 BULLETIN: MUSEUM OF COMrAKATIVE ZOOLOGY. 



It has been stated iu the account of the preceding cleavage that the 

 cell b^-^ does not always touch the anterior edge of the blastopore (see 

 Plate 6, Figs. 48 and 52), for the reason that tlie cleavage piano between 

 b^-^ and b^-* may vary in position from perpendicular to the long axis of 

 the egg to coincidence with the sagittal plane of the embryo. In any 

 event it seems certain that these two cells always form the anterior pair 

 of " secondary mesoblasts." In cases like that represented in Figures 

 48 and 52, the cells become shifted during the sixth cleavage, so that 

 the plane between them approaches coincidence with the sagittal plane 

 of the embryo — the common position of these cells in the thirty-two- 

 cell stage. 



The position of the posterior pair of "secondary mesoblasts" with 

 reference to the anterior pair and also to the blastopore leads to the 

 unavoidable conclusion that they are cut off from the cells a^"' and c®'^, 

 Avhich are at the sides of blastopore in the thirty-two-cell stage (Figs. 51, 

 52). These cells are represented in Figures 58 and 59 (Plate 7) as 

 dividing. From their position later, I infer that as division progresses 

 the extension of the blastoderm causes these cells to approach the median 

 plane, where the}"^ meet and complete the closing of the blastopore. At 

 the same time the primary mesoblasts d^'^, cP-^ are overgrown by the 

 blastoderm, and the cells a^-^ and c^'^ complete their division into the 

 outer cells (a^•^ c'-^), which remain in the blastoderm, and the inner cells 

 {d''^, c'-^), which constitute the posterior pair of " secondary mesoblasts," 

 lie between tlie blastoderm and the primary mesoblasts (see Plate 7, 

 Fig. 62 ; Plate 8, Fig. 72). 



Cases like those illustrated by Figures GO and Gl (Plate 7) give addi- 

 tional evidence in support of the above interpretation of the origin of tlio 

 " secondary mesoblasts." In the egg represented in Figure GO a rem- 

 nant of the blastopore is present and at its anterior edge are the two 

 blastoderm cells V-^, W-^. Immediately beneath them are the derivatives 

 b''-^ and b''"'', the anterior pair of " secondary mesoblasts." In the egg 

 represented in Figure 71 (Plate 8) the primary mesoblasts (c?"-', d'^-*) 

 have sunk beneath the blastoderm. The same relations exist between 

 blastopore and anterior " secondary mesoblasts." Similarly iu Figure 

 62 the posterior "secondary mesoblasts" lie beneath the cells a'-^ and 

 c'-*, which bound the sides of the blastopore. These cells are contigu- 

 ous to V-^ and 5''•^ The same relations hold in Figure 60 and in Figures 

 58 and 59 (Plate 7), which represent the divisions forming the " secon- 

 dary mesoblasts." Comparison of the arrangement of the cells around 

 the blastopore iu the thirty-two-cell stage (Plate 6, Figs. 51, 52) with 



