DEVELOPMENT OF PARACOPIDOSOMOPSIS 17 



there is an appreciable difference in the size of the two blasto- 

 meres, it is seen to pass with about equal frequency into the large 

 (fig. 15 and text fig, 1, A, H). and smaller cells (fig. 16 and text 

 fig. 1, D, G). It is found with equal frequency in the right- and 

 left-hand blastomeres as they lie on the sHde; but this would not 

 disprove homology any more than would size differences, for it 

 is evident that the position of an egg on the slide is a matter of 

 accident, so that what appears from above to be the right side 

 in one egg may correspond to the left side in another egg. The 

 point raised above cannot, therefore, be decided from a study of 

 two-celled stages. 



In the four-celled stage the typical condition shows the nucleo- 

 lar cell to be one of the two upper cells, which is invariably smaller 

 than any of the other three cells (text fig. 1, 1-K, 0, P). This 

 condition is found with very great frequency, and were it not 

 for certain variations, might easily lead one to conclude that the 

 nucleolus is handed on to a definite blastomere of the four-celled 

 stage. The most significant of these variations is the one showing 

 the nucleolus in one of the lower cells (fig. 15). It is impossible 

 to homologize the nucleolar cell in figm-e 18 with that in text 

 figure 1, K. 



The reason why the nucleolus is found so often in one of the 

 upper cells is not to be explained on the basis of homology, but on 

 entirely different grounds. I have already pointed out that as 

 the female pronucleus moves to the posterior end of the egg it 

 is followed by the nucleolus, and by the time the cleavage nucleus 

 is organized it comes to he close to this nucleus, usually to one 

 side and above, rarely below. In the two-celled stage the nucleo- 

 lus occupies this same relative position with reference to the nu- 

 cleus of its blastomere (figs. 14, 15). The cytoplasmic division 

 which produces the four-celled stage will result in placing the 

 nucleolus in an upper cell. If the nucleolus sinks below the level 

 of the equatorial plate (fig. 16), the resulting division will produce 

 a condition like that seen in figure 18. A study of many four- 

 celled stages shows that any one of the four cells may inherit 

 the nucleolus, but that it goes into one of the upper cells much 

 more frequently than into one of the lower cells. 



