108 M. Ussow's Zoologico-Emlryological Investigations. 



(" genninal spot," Kolliker) is produced. In this, as regards 

 the size and form of its constituent cells, and also their dis- 

 ti'ibution, the following two divisions may be distinguished : 

 — 1, the centre of the germinal disk, which presents the form 

 of a convex circle, and has been formed by the multiplication 

 of the high cylindrical primitive segmentation-cells (see the 

 stage of eight segments) ; and, 2, the originally very narrow, 

 but gradually widening ring, which immediately follows the 

 above-mentioned disk : the somewhat broader, but flatter, pen- 

 tagonal or hexagonal cells of which have been formed chiefly 

 from the apices of the segments constricted off by the meri- 

 dional furrow (see the stage of the meridional furrow). 



Directly united with this ring is the inferior part, which 

 extends to the inferior pole of the nutritive vitellus and 

 encloses the latter. This part consists of the apices of seg- 

 ments* slowly advancing in their division, and of the 

 segments themselves, which are here (at the inferior pole) 

 not sharply separated, but often even mutually coa- 

 lescent. Their number remains as before (thirty-two). 

 Their finely granular jn'otoplasm covers with a very thin 

 layer the whole mass of the nutritive vitellus, which in this 

 way is enclosed as in an envelope from the very commence- 

 ment of the segmentation in the so-called formative vitellus, 

 or, to be more exact, in the protoplasm of the primitive ovi- 

 cell, lying uniformly on its surface except at the superior pole, 

 where it is perceptibly thickened. The so-called disappear- 

 ance of the segments in reality never occurs. Earlier or later 

 they all divide, as we shall see, and furnish a certain number 

 of the cells forming the one-layered blastoderm f. 



From the actual course of the process of segmentation of the 

 Cephalopod ovum here described, and Avhich I have traced in 

 all its details, we may easily convince ourselves of the inac- 

 curacy of the opinion expressed by Kolliker upon this ques- 

 tion. And, in fact, I have perfectly convinced myself, by a 

 series of frequently repeated investigations, that he observed 

 stages in the development of the ova of Sepia which were 

 quito independent of each other, and that his researches were 

 can-ied on under abnormal conditions, in which the union of 

 the segments and the segmentation-apices w^as already much 

 injui'ed. Thus, for example, Kolliker indicates in the centre 

 of union of the segmentation-spheres indefinite and iiTcgular 



* In the last stage of segmentation the apex of every segment divides 

 into groups of cells, which arrange themselves in parallel rows on the 

 equator. 



t In fiepia the blastoderm closes at the inferior pole of the nutritive 

 vitellus only in the second period, as, indeed, Kcilliker has described. 



