Ryder.] \d& [May 16, 



average, when the chromatin is reduced to a volume equivalent to that of 

 the chromatin in a single spermatozoon of the same species. The tendency 

 towards the expulsion of polar bodies is therefore probably self-regulative 

 as soon as a certain minimum in the size of the chromatin masses is 

 attained. The impulse leading to such a result arises from the presence 

 of a large cytoplasmic field sensitive to external stimuli, but, in that such a 

 field is cut off from further possibility of growth by detachment from the 

 parent organism and incapable of further growth except through the 

 stimulus of its chromatin, and in that no more of the latter is for the time 

 being elaborated after detachment, it is clear that the cleavages which 

 give rise to the polar bodies are self-limited in number by conditions aris- 

 ing within the egg, and as a consequence of the specialization of the latter 

 as a cell, and in the sense that it differs from primitive types of cells as a 

 consequence of its method of protected growth within the parent. 



Why, however, should the polar bodies be so small ? Why does not the 

 egg divide equally ? This may be answered on the ground already as- 

 sumed that the chromatin is yet neither male nor female, but tends univer- 

 sally to be reduced to male dimensions even in the egg. The cytoplasm 

 being the most abundant in the egg and the chromatin in the spermato- 

 zoon, it is clear that totally ^different physiological characters must be 

 offered by the two elements. This, in fact, is the essence of the meaning 

 of the term specialization as applied to them, and involves the conception 

 of wide differences in the modes in which physiological energy has worked 

 to produce them, respectively. If the yolk is abundant, the cytoplasm, 

 at one pole of the egg where nuclear cleavage occurs most readily to 

 form the polar bodies, is reduced to a thin layer or disk. This, in many 

 cases, is the condition under which polar bodies are produced so that a 

 great inequality in the size of the cleavage products must result. Later, 

 when the egg nucleus is reduced and can return to a deeper position in the 

 egg, it can gain control of a still larger cytoplasmic field, which is still 

 further enlarged by the advent of a fresh male chromatin and cytoplasmic 

 element. When the male and female elements finally unite there is a com- 

 plete readjustment of the equilibrium between the cytoplasm and chroma- 

 tin centres, because the introduced male is capable of taking control of a 

 still larger cytoplasmic field and may even at times overtop the female, as 

 in the case of Rhynchelmis described by Vejdowsky. The two together 

 now regain control of the cytoplasmic field of the egg, but cut off from 

 direct dependence upon the parent, so that a new cycle of changes can go 

 on in a new way, and instead of running down towards the male condi- 

 tion, normal segmentation goes on which ends in the formation of a 

 new being under the impulse of the tendencies towards continuous growth 

 under new conditions. The cases of egg and spermatozoon are clearly 

 merely specialized states of chromatin and cytoplasm and their separated 

 and united conditions are merely phases of a continuous process of growth 

 under widely differing conditions which are ushered in as the results, first, 

 of an incipient and complete exclusion from the parent (formation of polar 



