136 



THE EVOLUTION OF SEX. 



with each species ; what takes place in the act of fertilization, however, is 

 always essentially the same. The head of the spermatozoon becomes the male 

 nucleus (or pro-nucleus) of the fertilized ovum, entering into close association 

 with the female nucleus. The latter, as we have already noted, has had its own 

 history ; it is no longer the original germinal vesicle, nor usually like it in 

 appearance, it is the germinal vesicle minus the quantity of nuclear substance 

 given off in forming two polar globules. This female nucleus (or pro-nucleus, as 

 it is generally called) comes into close association with the sperm or male 

 nucleus ; nor does it remain quite passive in the process, though the greater 

 activity in bringing about the close association is certainly still exhibited by the 

 male. Whitman has recently emphasized the reality of an attractive influence 

 \ between the pro-nuclei. Fusion of the pro-nuclei was observed so long ago as 

 1850 by Warneck in the pond-snail (Lymnczus). This result, however, appears to 

 have been overlooked, till the same fact was reobserved in threadworm ova by 

 Biitschli in 1874. Since that date the fact has been continuously studied. Some 

 observers still doubt whether what can be accurately called fusion of nuclei ever 

 occurs ; and if fusion means inextricable confounding and mixing up of the male 

 and female nuclear elements, it is almost certain that such does not in any case 

 happen. There is no doubt, however, that the two nuclei become very closely 

 associated, and according to most observers a double unity is formed, in which 

 the component nuclear elements from the two origins so diverse are united in 

 perfectly orderly fashion. So exact, in fact, is this duality, that when the first 

 division of the egg takes place, each of the two daughter-cells has in its nucleus 

 half of the male and half of the female elements, and so on perhaps in after- 

 stages. 



The object upon which the intimate phenomena of fertilization have been 

 most studied is the ovum of the threadworm {Ascaris megalocephala) which 

 infests the horse. Since 1883 about a dozen important memoirs have dealt with 

 this subject, and with the same material. The results of competent observers 

 have varied enormously in detail, but on the essential points there has been 

 (with some few exceptions) an increasing congruence of opinion. The most 

 important work on the subject has been that of Prof, van Beneden, whom most 

 of those who have followed him unite in regarding as a master. The 

 discrepancies and contradictions have been accompanied at times by not a little 

 warmth of asseveration, but with ever-increasing perfection of method many of 

 these are disappearing. To one alone shall we here allude. According to Van 

 Beneden, the normal ovum of this threadworm contained in its nucleus one 

 chromatin element, and was fertilized by a sperm also with one chromatin 

 element. Carnoy, however, described the normal ovum as containing two 

 chromatin elements, and as fertilized by a sperm also with two. In view of the 

 perfection with which both these investigaters had unravelled the structure and 

 behavior of the nuclei, the discrepancy seemed serious enough. Now, however, 

 Boveri has shown that both are right ; Van Beneden 's type occurs ; Carnoy's 

 type also occurs. Nay more, an ovem with one chromatin element seems to be 

 always fertilized by a sperm with only one, while an ovum with two chromatin 

 elements is fertilized by a sperm likewise with two. 



A few of the details may be summarized from the recent masterly researches 

 of Boveri. The extrusion of the two polar cells from the ovum is in reality a 

 double process of cell-division. The quantity of the nuclear substance in the 

 germinal vesicle is thereby reduced by three fourths, but the number of nuclear 

 elements remains the same. Only one sperm penetrates the ovum, unless the 

 latter be unhealthy ; and with the entrance of the sperm the ovum undergoes a 



