394 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



yolk formation and to the great growth of the cytoplasm by which this latter 

 process is accompanied. 



The progressively increasing basophile reaction of the cytoplasm which pre- 

 cedes yolk formation is due to the gradual loading of the cytoplasm with material 

 supplied by the chromatin and passed out from the nucleus by diffusion through 

 the nuclear membrane, the material so supplied being subsequently utilized in 

 yolk formation. It is not unreasonable to suppose that this progressive change 

 in the chemical constitution of the cytoplasm is accompanied also by a progressive 

 change in its physical consistency, a change resulting either in an increased or 

 decreased viscosity. The assumption that the change is in the direction of a more 

 fluid consistency is the one which seems most easily correlated with the changes 

 in the yolk immediately to be described. 



In the young egg the nucleolar spherules lie stationary and unchanged, slowly 

 accumulating in the faintly staining cytoplasm, the viscid nature of which is 

 indicated by the arrangement of the spherules in small clusters, each representing 

 a discharge from the nucleolus, and each having penetrated only a little way 

 into the cytoplasm. With the appearance of the basophile reaction and, as Pro- 

 fessor Chubb assumes, more fluid consistency of the cytoplasm, the nucleolar 

 spherules can no longer retain their original character, and the substance of which 

 they are composed diffuses onto the surrounding cytoplasm. At first the areas of 

 the cytoplasm so affected are scattered and diffuse in character. Gradually, how- 

 ever, they come together and assume a form in contact with the nucleus, which is 

 entirely in agreement with the assumption that capillary laws constitute the sole 

 morphogenic factor, the presence of the substance of the nucleolar spherules hav- 

 ing so modified the physical character, and hence also the surface tension, of the 

 region of the cytoplasm which bears it, as to give it a physical individuality of 

 its own. 



By the continued change in the physical consistency of the cytoplasm which 

 accompanies the increase in its basophile reaction, and by the yet further addition 

 of nucleolar substance to the yolk nucleus, the difference in surface tension between 

 these two regions of the cytoplasm is still further augmented, and the yolk nucleus, 

 leaving the surface of the nucleus, assumes a more compact and rounded form 

 midway between the nucleus and the egg periphery, often, indeed, being almost, 

 in contact with both. 



Yolk formation now commences, and as this progresses the coarsely alveolar 

 structure it produces causes the cytoplasm to lose much of its original semifluid 

 consistency. Capillary forces are therefore no longer able to exert the influence 

 they formerly possessed in maintaining the regular shape of the yolk nucleus, and 

 the smooth contour of the latter gives place to a somewhat irregular outline. 



During the actual process of yolk formation there is a rapid and very con- 

 siderable increase in the bulk of the cytoplasm, the diameter of the egg being often 

 more than quadrupled during the process. As the yolk nucleus represents an 

 area of the cytoplasm only differing from the rest by being infiltrated with the 

 substance of the nucleolar spherules, it also should show an increase in bulk corre- 

 sponding to that of the whole egg. This increase in the size of the yolk nucleus 



