No. 3] STUDIES ON CEPHALOPODS. 283 



which the achromatic radiating fibrils abound. Next to this is 

 the zone of granular cytoplasm which passes rather suddenly 

 into the zone of the thinner protoplasmic pellicle, a portion of 

 which is shown in the figure. The whole mass of the yolk- 

 substance is enveloped by the extension of this same pellicle. 



A further study of the same blastoderm will show that the 

 nucleus is not situated exactly in the centre of the germ-disc, 

 but it is slightly eccentric. Taking the outermost limit of the 

 granular zone as the contour of the essential part of the devel- 

 oping egg, the cleavage nucleus is found situated nearer to one 

 side "than to the other. I have made a series of careful meas- 

 urements in regard to this point, and found in a large majority 

 of cases, this eccentric position of the nucleus in the blastodisc 

 is well pronounced, and even discernible without any aid of a 

 micrometer. This fact was already noticed by Vialleton in 

 Sepia. Comparing this observation on the isolated blastoderm 

 with the surface study, we notice an interesting coincidence 

 of the two. For if we draw a straight line through the cleav- 

 age nucleus, transverse to the germ-disc, as it is represented 

 in the plate, it will divide the latter into two unequal halves, 

 the one lying in front of the line being larger than the one lying 

 behind it, while if we draw a straight line at right angles to 

 the first and through the same nucleus, it will divide the disc 

 into two identical bilateral halves. The larger segment lying 

 in front of the transverse line corresponds to the anterior 

 half, and the smaller segment behind the line corresponds to 

 the posterior half of the ovum. The two bilateral identical 

 segments falling on both sides of our second imaginary line 

 correspond to the right and the left half of the ovum. 



Bearing these points in mind, we will proceed to the examina- 

 tion of the different stages of cleavage, with special reference 

 to the development of symmetry as manifested by the cleavage 



furrows. 



Fig. 17 shows a stage in which the caryokinetic figure of the 

 cleavtge nucleus has just reached a stage where the chromatic 

 contents of the nucleus have been reduced into a " plate," and 

 the general outline of the spindle has become completed. The 

 spindle is formed entirely within the original nuclear cavity, 

 which is sharply separated from the cytoplasmic surrounding. 

 The archoplasmic rays of the aster run in all directions, and 



