lOG J. Thos. Patterson. 



wenige vereinzelte Zellen vor."^° I fully concur with Hertwig's 

 views regarding the fate of the non-nucleated yolk masses, for one 

 can examine scarcely a series in which some evidence of their dis- 

 integration is not found. The manner in which these masses break 

 up is of interest, in that the fragments often resemble cells. Thus, 

 above and to the left of the mass at dm. Fig. 47, are smaller masses 

 that have broken off and become spherical — a process probably com- 

 parable to the phenomena of surface tension. These smaller masses 

 in turn continue to subdivide until the cavity may become crowded 

 with very fine particles (Fig. 44) which in this state are doubtless 

 taken up by the cells. The yolk masses therefore play no role in 

 the formation of the primary germ layers, except, of course, indirectly 

 as nutriment. The contention of Balfour ('73) that they may 

 become nucleated by the formation of nuclei de novo from yolk 

 spherules would scarcely accord with the views of modern cytologists. 



Again, in regard to the significance of the nucleated elements with- 

 in the cavity, I agree with Hertwig, who thinks that in numbers 

 they are far too few to be of any importance in the formation of the 

 germ layers. On making counts of these elements, I was surprised 

 to find that in blastoderms such as shown in Fig. 47, less than two 

 per cent of them are nucleated, and that even in this small number 

 many of the nuclei show signs of degeneration (Fig. XIV, B). !N^ot 

 infrequently the cytoplasmic portion of such elements disintegrate, 

 leaving the nucleus lying free within the cavity (Fig. 52, n). In 

 other cases neither the cytoplasm nor nucleus breaks down at first, 

 but the latter multiplies at the expense of the former until a solid 

 mass of nuclei is formed (Fig. 55). Sooner or later these nuclei 

 go to pieces. 



These abnormal nuclei are to be accounted for by the fact that 

 some of the yolk masses in arising from the central periblast (Fig. 

 46) naturally take up the periblastic nuclei, which, as was shown 

 above, are degenerating. Their presence is in no way necessary to 

 the formation of the masses, as is evident from the fact that the 

 large majority of the masses are non-nucleated. 



The phenomenon of yolk mass formation is only an index to the 



'^Loc. cit., p. 858. 



