342 CAUSES OF VARIATION 



parts of the body), the second setting off dorsal and ventral 

 areas ; while the third, which is horizontal, divides into anterior 

 and posterior portions. 



The process is the same in many species, and " wherever the 

 egg axis can be determined by the accumulation of the deuto- 

 plasm in the lower hemisphere the egg nucleus sooner or later lies 

 eccentrically in the upper hemisphere, and the polar bodies are 

 formed at the upper pole." 



In such cases of distinct polarity the cleavage planes are prac- 

 tically predetermined, and the products of division have each 

 their definite r61e in development. Thus the upper hemisphere 

 (so-called animal pole) is the seat of most active division, with 

 smaller cells, which develop into cerebral, dorsal, and anterior 

 portions of the body ; while the lower hemisphere (vegetable or 

 nutritive pole) divides more slowly, its cells are larger, and they 

 develop into the alimentary organs and the posterior and ventral 

 parts generally. 1 



While this rule is not absolute in all species, it yet indicates a 

 broad general principle that lies at the very threshold of devel- 

 opment ; namely, that the original impulse to direction of growth 

 lies in the polarity of the ovum. 



Cause of polarity in the ovum. Gravity would seem to be the 

 controlling cause in establishing a kind of polarity in the ovum. 

 The nucleus being of low specific gravity, tends to lie eccentrically 

 nearer the upper side of the ovum, and the heavier deutoplasm 

 settles to the lower side, the parts arranging themselves accord- 

 ing to relative weight, like starch granules or other cell contents. 

 Moreover, Born has shown that " if frogs' eggs be fastened in 

 an abnormal position, e.g. upside down or on the side, 

 rearrangement of the egg material takes place " and " a new axis 

 is established in the egg." 2 Schultze discovered that ".if the egg 

 be turned upside down when in the two-celled stage a whole 

 embryo (or half of a double embryo) may arise from each blasto- 

 mere, instead of a half embryo as in the normal development, 

 and that the axes of these embryos show no constant relation 

 to one another." 2 Morgan learned that " either a half embryo 



1 Wilson, The Cell, pp. 378, 379. 



2 Ibid. p. 422. 



