30 BIOLOGICAL LECTURES. 



formation of the trochoblasts and of the basal and tip cells 

 in the arms of the cross, the initial eccentricity of the nuclear 

 spindle indicates that here also there must be some difference 

 of material substance within the cell, though not directly- 

 visible. Sachs^ has well said, "The external form as well as 

 the internal structure of any body are the necessary expres- 

 sion of its material constitution. Difference in form always 

 indicates difference of material substance." That the cause 

 of unequal cleavage is more complex than the mere mechani- 

 cal displacement of the nuclear spindle is proven by the fact 

 that the first two divisions of the egg are frequently equal, 

 though the polar differentiation of the protoplasm and yolk is 

 as marked as in later divisions which are very unequal. 



What and how many factors enter into the complex of causes 

 which produce even such simple phenomena as non-alternating, 

 non-rhythmical, and unequal cleavages it is at present impos- 

 sible to say; however, the prospective significance, the "pur- 

 posefulness," of such cleavages is often very apparent. Lillie^ 

 has pointed out the fact that unequal cleavages in Unio stand 

 in direct relation to the size of the parts arising from the blas- 

 tomeres. With the following slight modification this principle 

 is applicable to all the gasteropods which I have studied, viz., 

 the initial size of the blastomere stands in direct relation to the 

 size and the time of formation of the part to which it gives 

 rise. In fact, this is but a partial expression of a much more 

 general truth, z'/^., that all differential cleavages, whether non- 

 alternating, non-rhythmical, or unequal, are directly and causally 

 related to the uses to which these cells are put, — in short, to 

 the general differentiation of the organism. 



Other attempts have been made to explain the definite rela- 

 tion between blastomeres and organs than the one here given, 

 viz., that the differentiation of the blastomere stands in direct 

 relation to the differentiation of the parts and that the former 

 is the result of differences in the material constitution of the 

 cells. Hertwig^ ascribes the fact that organs may be traced 



i Sachs, J. v., "Physiology of Plants," Lecture XLIII, 1882. 



2 Lillie, F. R., "The Embryology of the Unionidae," Journal of Morphology, 

 X, 1895. 



3 Hertwig, O., " Urmund und Spina-bifida," Arch, fitr mik. Anat., Bd. 39, 1892. 



