286 CLEAVAGE (segmentation) and blastulation 



pattern and the rate of blastomere formation is an internal one resident in 

 the organization of the egg and the peculiar protoplasmic substances of the 

 various blastomeres. This apparent fact suggests strongly that the egg in its 

 development "is a builder which lays one stone here, another there, each of 

 which is placed with reference to future development" (F. R. Lillie, 1895, 

 p. 46). 



c. Influence of First Cleavage Amphiaster on Polyspermy 



In figure 138 is shown the behavior of the sperm nuclei during fertiliza- 

 tion in the urodeie, Triton. This figure demonstrates that the developing first 

 cleavage amphiaster suppresses the development of the accessory sperm nuclei. 

 Similar conditions appear to be present in the elasmobranch fishes, chick, 

 pigeon, etc. 



d. Viscosity Changes During Cleavage 



"The viscosity changes that occur in the sea-urchin egg are probably typical 

 of mitosis in general. There is marked viscosity increase in early prophase, 

 then a decrease, and finally an increase just before the cell divides" (Heilbrunn, 

 '21 ). Similarly, Heilbrunn and W. L. Wilson ('48) in reference to the cleaving 

 egg of the annelid worm, Chaetopterus, found that during the metaphase of 

 the first cleavage the protoplasmic viscosity is low, but immediately preced- 

 ing cell division protoplasmic viscosity increases markedly. 



e. Cleavage Laws 



Aside from the factors involved in cleavage described above, other rules 

 governing the behavior of cells during division have been formulated. These 

 statements represent tendencies only, and many exceptions exist. "The rules 

 of Sachs and Hertwig must not be pushed too far" (Wilson, E. B., '25, p. 985 ) . 



1) Sachs' Rules: 



(a) Cells tend to divide into equal daughter cells. 



(b) Each new cleavage furrow tends to bisect the previous one at right 

 angles. 



2) Hertwig's Laws: 



(a) The typical position of the nucleus tends to lie in the center of the 

 protoplasmic mass in which it exerts its influence. 



(b) The long axis of the mitotic spindle typically coincides with the long 

 axis of the protoplasmic mass. In division, therefore, the long axis of 

 the protoplasmic mass tends to be cut transversely. 



6. Relation of Early Cleavage Planes to the 

 Antero-posterior Axis of the Embryo 



In the protochordate, Styela, the first cleavage plane always divides the 

 yellow and gray crescent material and other cytosomal substances into equal 



