1890.] [Ryder. 



growth of the ovum? This, I believe, may be answered on the supposi- 

 tion already to some extent elaborated that the egg is an abortive attempt 

 at the production of an overgrown spermatogonium which is set free 

 before it has been fully matured, as a result of the precocious determina- 

 tion of superabundance of surplus nutriment to it. 



This has been due to forces operating within the parent organism ; 

 how, we are still unable to clearly state. If this is so, then the speciali- 

 zation of the egg is accounted for and the expulsion of the polar bodies 

 may be approached from another point of view, namely, that of their 

 morphological equivalence to spermatozoa, since they represent largely 

 the characteristically male plasma in their chromatin. The egg is, there- 

 fore, specialized in so far as it is an abortive spermatogonium, and the num- 

 ber of polar bodies, produced as abortive spermatic elements, represent its 

 degree of specialization. The consequent reduction of the chromatin in 

 the egg nucleus may then also be compared with the processes of spermato- 

 genesis in which a certain minimal size of the chromatin mass of the egg 

 is reached, which now makes the ovum the exact homologue of the 

 spermatozoon, but with an enormous cytoplasmic body fitted for the exhi- 

 bition of active karyokinetic movements and an elaborate series of suc- 

 cessive and finally simultaneous karyokineses. 



In this way it may be supposed that the peculiar advantages offered for 

 the survival of a species through sexual processes may be realized.* But 

 such advantages were developed not as the result of any foresight, but as 

 a consequence of the action of the principle of overnutrition ending in 

 the production of spermatogonia which failed to segment or break down 

 into male elements before they were freed from the parent. In this way 

 it may be supposed that the ovum itself arose, but that it was a later phase 

 of development than that of the flagellate male germs, which type still 

 prevails in asexual or very primitive forms. This gives us the real 

 grounds for the evolution of the ovum ; accounts for its specialization, for 

 the reduction in volume of its chromatin to that of the male element 

 through the expulsion of the polar bodies, through which it also again 

 becomes the immobile overgrown, but exact morphological homologue of 

 the spermatazoon. The specialization which the ovarian egg has attained 

 as an overgrown spermatogonium also makes it certain that the cells ex- 

 pelled as polar bodies represent the energy in part which has been 

 expended, and which is signified by the great size of the ovarian egg. 

 These products of specialized development must be got rid of so that this 

 part of my hypothesis respecting the polar bodies is a necessary corollary 

 of the first part developed in the earlier portion of this paper. 



The impulse towards the expulsion of the polar bodies comes from with- 

 in, upon the advent of an adequate stimulus, and the tendency is to run 

 down towards the male condition from the egg, but such a result is pre 

 vented from proceeding far by the small original amount of chromatin in 

 the egg which prevents the formation of more than two cleavages, on the 



* "Origin and Meaning of Sex." Am. Naturalist, June, 1889, pp. 501-508. 



