No. 3.] STUDIES ON CEPHALOPODS. 289 



20 and 21, has divided into 16 segments, while each segment 

 in the posterior half has divided into 14. We must also bear in 

 mind that the size of the inner cells in the posterior half is gen- 

 erally smaller than that of the anterior ones, showing that the 

 original anterior half of the blastoderm is ahead of the poste- 

 rior in the number of segments as well as in the actual bulk 

 of the protoplasm. An examination of the caryokinetic figures 

 will show that the segments on the left side are further advanced 

 than those on the right side. 



It is a difficult matter to decide of just how many segments a 

 given stage of the blastoderm consists, when the division of 

 cells is such a gradual process, and when there exist stages 

 showing perfect transition from one condition to another. Such 

 a stage is shown in Fig. 31. The number of segments differs 

 according to the kind of criterion one takes in deciding what 

 constitutes a completion of cell-division. One thing is, however, 

 certain, — that the growth of the blastoderm, as indicated by 

 the conditions of the nuclei in different parts, is not uniform. 

 The right side of the figure is decidedly ahead of the left half, 

 and the left quadrant in the anterior half of the blastoderm is 

 decidedly behind the right quadrant in the same half. 



Fig. 32 represents the blastoderm at the 1 16 cell stage. The 

 left half of the blastoderm is in advance of the right half, 

 although their line of demarcation does not exactly coincide 

 with the plane of the first cleavage. The cells situated in the 

 central portion of the blastoderm are backward in growth and 

 division, as in all preceding cases. 



The numbers around the marginal cells indicate the order of 

 succession of cleavage furrows in the marginal zone alone, in 

 this case, as well as in all of the preceding specimens we have 

 described. 



V. 



By examining the stages of division more carefully, we find 

 that in many cases the divisions of the different segments in 

 the same ovum do not take place at the same time and with the 

 same velocity. We further find a curious fact, that when one 

 segment or a few segments on one side of the blastoderm show 

 a tendency to vary in a certain particular direction, the corre- 

 sponding segment or segments on the opposite side show the 

 same tendency. 



