No. 3.J STUDIES ON CEFHALOPODS. 287 



Fig. 26 represents a blastoderm with 30 segments, with 12 

 inner cells with 18 marginal cells. Two of the inner cells have 

 nearly completed division, but each has been counted as one. 

 Most of the cells are in a resting condition, and the rate of 

 growth is uniform on both sides of the blastoderm. 



Fig. 27 represents the 32 cell stage, with 14 inner cells and 18 

 maririnal cells — 16 cells on each side of the first or the second 



o 



cleavage furrow. 



If one, however, divide the blastoderm into front and behind 

 by the furrow of the first cleavage, as is clearly indicated by the 

 state of nuclear changes, we find an interesting contrast in 

 the division of the inner and marginal segments. For while we 

 find 10 marginal segments out of the entire 16 of the blastoderm 

 in the front half alone, we find only 8 in the posterior half. In 

 the quantity of the inner segments, however, the posterior half 

 is ahead of the anterior; for the former contains 8, and the 

 latter, only 6. Hence, while the anterior half is ahead of the 

 posterior in the number of marginal cells, the latter is ahead of 

 the former in the number of the inner cells ; and hence the 

 numerical superiority of the anterior half of the blastoderm is 

 produced by cells of larger dimension than the cells in the 

 posterior half, the anterior half of the blastoderm occupying a 

 larger area than the posterior half. Besides the 6 inner cells in 

 the anterior half occupy a larger area than the area covered by 

 8 inner cells in the posterior half, altogether making the devel- 

 opment of the front half more conspicuous than that of the 

 posterior half. 



Fig. 28, PI. XII, shows a 32 cell stage — 16 in front and 16 

 behind the second cleavage furrow (2' -2). The first cleavage 

 furrow (I'-i) divides the blastoderm into two bilateral halves, of 

 which the left side is more advanced than the right side, as the 

 conditions of their nuclei clearly show. The numerical propor- 

 tions of the marginal and the inner cells in the posterior and 

 anterior halves are exactly the same as in Fig. 27 (PI. XI), 

 described already. The development of a pair of marginal 

 segments in Fig. 28, the segments lying between 2' and 4, and 

 2 and 4 respectively, on each side of the blastoderm, is con- 

 siderably different from the development of the corresponding 

 segments in Fig. 27 or Fig. 26. In Fig. 28 the marginal seg- 

 ment (4-2') does not reach the margin of the blastodisc at all, 



