GENERAL CONSIDERATIONS 285 



of cleavage. Undoubtedly, in many instances, the accumulation of yolk ma- 

 terials does impede or alter the cleavage furrows, although it does not suppress 

 mitotic divisions of the nucleus as shown in the early cleavages in many 

 insects, ganoid fishes, etc. On the other hand, the study of cleavage phe- 

 nomena as a whole brings out the fact that other intrinsic factors in the 

 cytoplasm and organization of the egg largely determine the rate and planes 

 of the cleavage furrows. 



2) Organization of the Egg. An illustration of the dependence of the 

 pronuclei and of the position of the first cleavage amphiaster upon the general 

 organization of the cytoplasm of the egg is shown in the first cleavage spindle 

 in Amphioxus and Styela. In the eggs of these species, the amphiaster of the 

 first cleavage always orients itself in such a way that the first cleavage plane 

 coincides with the median plane of the future embryonic body. The first 

 cleavage plane, consequently, divides the egg's substances into two equal 

 parts, qualitatively and quantitatively. The movements of the pronuclei and 

 the first cleavage amphiaster are correlated and directed to this end. 



Various theories have been offered in the past to account for the migrations 

 of the pronuclei at fertilization and for the position of the first cleavage 

 amphiaster. All of them, however, are concerned with the cytoplasm of the 

 egg or its movements, which in turn are correlated with the organization of 

 the egg. (See Wilson, E. B., '25, p. 426.) 



A second illustration of the dependence of the chromatin-amphiaster com- 

 plex on conditions in the cytoplasm is afforded by experiments of Hans 

 Driesch in 1891 on the isolation of the blastomeres of cleaving eggs of the 

 sea urchin. He found that the first cleavage of the egg occurred from the 

 animal to the vegetal pole, resulting in two blastomeres. Now, if these blas- 

 tomeres are shaken apart, the following cleavages in the isolated blastomeres 

 behave exactly as if the two blastomeres were still intact, indicating a definite 

 progression of the cleavage planes. That is, there is a mosaic of cleavage 

 planes determined in the cytoplasm of the early egg. 



A third example of the influence of egg organization upon cleavage is 

 afforded by the egg of higher mammals. In this group, the first cleavage plane 

 divides the egg in many cases into a larger and a smaller blastomere. The 

 larger blastomere then begins to divide at a faster rate than the smaller 

 blastomere. This accelerated division is maintained in the daughter cells re- 

 sulting from the larger blastomere. Here, then, is an egg whose yolk material 

 is at a minimum. Nevertheless, the blastomeres which result from the first 

 cleavage are unequal in size, and the cellular descendants of one of these 

 blastomeres divide faster than the descendants of the other blastomere. Some 

 conditioning effect must be present in the egg's cytoplasm which determines 

 the size of the blastomeres and the rate of the later cleavages. Many other 

 illustrations might be given from the studies on cell lineage. However, the 

 conclusion is inevitable that under normal conditions the cause of the cleavage 



