358 PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 



liyaline protoplasm, and separating off by kaiyokinetic division. After 

 mtituration is complete, a single spermatozoon normally penetrates; 

 then a reaction immediately sets in in the cell wall of the ovum which 

 prevents other spermatozoa from entering. The head of the spermato- 

 zoon and the nucleus of the ovum now fuse together to form a single 

 nucleus, which it is obvious contains the hereditary substance of two 

 individuals. This is the starting point of the segmentation or distri- 

 bution process above described, and it follows tliat the fertilized ovum 

 at this stage must contain its typical complement of chromatin, archo- 

 l>lasm, etc., for the whole course of growth to the adult. 



How shall we connect these phenomena of fertilization with the 

 facts of heredity? The most suggestive enigma in connection with 

 the fei'tilization process has been the meoning of tJie two polar bodies, 

 especially since Van IJeneden demonstrated that they contained chro- 

 matin? ]^\)r twenty-five years, speculation has been rife as to why the 

 ovum should extrude a portion of its substance in two small cells; why 

 not in one cell ? why not in a larger numl)er? Thanks to the intense 

 curiosity which these i^olar bodies have aroused, and to the great va- 

 riety of explanations which have been offered for them, we have ar- 

 rived to-day at a solution which links the higher animals with the 

 lower, breaks down the supposed barrier between the sexes, and ac- 

 cords with the main external facts of hereditj'. 



It seems to me best to disregard the order of discovery, and to state 

 the facts in the most direct way. First, a few words as to the specu- 

 lations upon the meaning of the polar bodies. 



The early views of fertilization* were naturally based upon the ap- 

 parent significance of this process in the human species, in which the 

 sexes are sharply distinguished from each other in their entire struc- 

 ture, and the reproductive cells are also widely differentiated in form, 

 the ovum large and i)assive, the spermatozoon small and active. Tlie 

 readiest induction was to regard these elements as representing dis- 

 tinct T)hysiological xninciples, corresponding to the essential sexual 

 characteristics — in short, as male and female cells, the former vitalizing 

 and rejuvenating the latter. Thus one of the earliest definite "polar- 

 body" theories was that the ovum was hermaphrodite, containing both 

 male and female princii)les, and that it was necessary to get rid of ilie 

 male substance before the spermatozoon could enter. 



As Von Siebold and Leuckart had demonstrated that some ova re- 

 produce parthenogenetically, that is without fertilization by spermato- 

 zoa, Weismann turned to such forms for the solution of this problem, 

 and Avas surprised to find that parthenogetic ova only extrude one 

 polar body. This led him to attach one meaning to the first polar body, 

 and another meaning to the second, which he viewed as designed to 

 reduce the heredity substaiu;e in the ovum without regard to sex. 

 iThus both this and the older theoiy conveyed alike the idea of reduc- 



^jj .*S^e also the iutroduotiou of Weismann's last essay, "Ampliimixis." 



