70 CLEAVING OF THE YOLK 



ring of globules with cells is then formed, whereupon these and the 

 radiating segments again suffer division in the direction of the radius of a 

 circle. This process is repeated; the radiating segments meanwhile 

 extending at the periphery, while they are shortened and narrowed at 

 their apices by the divisions they undergo. At length these divisions 

 reach a stage, when the apex of each radiating segment is given off 

 together with the embryonic cell, leaving no remaining cell in the apices. 

 When this takes place, the radiating segments disappear, or cease to be 

 distinguishable from the rest of the yolk. Henceforward the germinal 

 space contains only globular segments of yolk, each including an em- 

 bryonic cell ; the globules being smallest in the centre. Kolliker maintains 

 that neither the earlier divisions of the yolk, nor even the last small 

 globules, can be regarded as cells. For the segments and the first formed, 

 larger, globules are, he says, merely hillocks of elementary granules on 

 the surface of the yolk collected around the embryonic cells. And the 

 small globules appear to differ from these only in their size, and in con- 

 taining more granular matter. In the Sepia, as in the Ascarides, Kolliker 

 is of opinion that the division of the yolk into segments or globules, is 

 dependent on some attractive force, exerted by the nucleated embryonic 

 cells upon the substance composing the yolk, and that the cells themselves 

 multiply, as in the Ascarides, by the development of two young cells within 

 each parent-cell. 



III. The third variety of the changes which take place in the yolk in 

 invertebrate animals, subsequently to fecundation and the disappearance 

 of the germinal vesicle, is exemplified in the ovum of Ascaris dentata ; 

 and the observations which Kolliker* has made on the changes of the 

 yolk in this Entozoon are of great importance, since, owing to the almost 

 complete transparency of the ova and their coats, it is scarcely possible 

 that any error of observation can have been committed. In these in- 

 vestigations Kolliker found that, for a short time after the disappearance 

 of the germinal vesicle, the ovum is seen to contain merely the trans- 

 parent fluid of the yolk, with very scanty elementary granules. But 

 soon the first embryonic cell is developed in the centre of the yolk. This 

 cell is quite transparent, globular, somewhat larger than the previous 

 germinal vesicle, and has a small, pale, round nucleus attached to its wall. 

 As the ova advance through the cavity of the uterus, this embryonic cell 

 is replaced by two similar cells; then four such cells are seen, then 

 eight, and so on, the cells becoming more and more numerous, and 

 diminishing in size as they increase in number. While these cells are 

 thus undergoing multiplication in the centre of the yolk, the yolk itself 

 suffers no change except in quantity ; it gradually disappears, being con- 

 sumed, as it would seem, in the growth of the embryonic cells. These, 

 while they multiply, occupy more and more space, for although each of 



* MUller's Archiv. 1843, pp. 7685. 



