No. 3.] STUDIES ON CEPHALOPODS. 285 



previous stage, becomes still more emphasized in the arrange- 

 ment of the resulting nuclei. For the sake of convenience, I 

 will call the segments in front of the second furrow of cleavage 

 the anterior, and those behind it the posterior, segments of the 

 blastoderm. The nuclei of the anterior two segments take dif- 

 ferent positions from those of the posterior ones. This is shown 

 by the attitude they assume in reference to the median plane, as 

 will be seen in the figure. 



The antero-posterior differentiation of the blastoderm is made 

 still more manifest in the succeeding stage. Fig. 21. The longi- 

 tudinal axis of the caryokinetic figure in each of the posterior 

 segments is almost parallel with the second furrow of cleavage, 

 while the same axis in the anterior segment makes an angle of 

 about 45° with it. 



The antero-posterior differentiation indicated by the disposi- 

 tion of the caryokinetic figure in the previous stage is made 

 still more apparent by the appearance of the cleavage furrows, 

 Fig. 22. While the third furrows in the posterior segments run 

 nearly parallel to the first and nearly at right angles to the sec- 

 ond cleavage plane, those in the anterior segments take quite a 

 different direction, as will be seen in the figure. They run 

 parallel to, or even converge towards the median axis in the 

 posterior half, while in the anterior half of the blastoderm they 

 run away from the median plane. 



In Fig. 23 the twelve segment stage of the blastoderm is 

 shown. No two cells on one side of the blastoderm are alike, in 

 size, in shape, or perhaps in their destiny. Just where the suc- 

 ceeding cleavage furrows come in, is indicated by the equa- 

 torial plane of the caryokinetic figure. 



Fig. 24 represents the sixteen segment stage of the blasto- 

 derm. The bilateral arrangement of the segments is perfect. 

 One important fact which will be discussed later may here be 

 pointed out ; viz. all the segments in front of the first cleavage 

 furrow are in a more advanced stage of caryokinesis than those 

 situated behind. This difference in the rate of growth in the 

 anterior and posterior halves of the blastoderm is most pro- 

 nounced in the behavior of four central cells, surrounded by 

 twelve marginal cells. Two central segments which are situated 

 anteriorly are considerably larger than the other two central 

 segments situated behind. The former belong to the original 



