SEXUAL REPRODUCTION. I45 



man has recently emphasised the reahty of an attractive influence between 

 the pro-nuclei. P\ision of the pro-nuclei was observed so long ago as 1850 

 by Warneck in the pond-snail [Lyinuccus). This result, however, appears 

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

 ova by Butschli 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 wdiich the component nuclear 

 elements from the tw^o origins so diverse are united in perfectly orderly 

 fashion. So exact, in fact, is this duality, that v/hen 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 fertilisation have been 

 most studied is the ovum of the threadworm {Ascaris uiegalocepJiaJd) 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 fertilised by a sperm also 

 with one chromatin element. Carnoy, however, described the normal 

 ovum as containing two chromatin elements, and as fertilised by a sperm 

 also with two. In view of the perfection with which both these investi- 

 gators had unravelled the structure and behaviour of the nuclei, the dis- 

 crepancy 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 ovum with one chromatin element seems to be always 

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

 elements is fertilised by a sperm likewise with two. 



A few of the details may be summarised 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 pene- 

 trates the ovum, unless the latter be unhealthy ; and with the entrance of 

 the sperm the ovum undergoes a simultaneous change, which excludes other 

 male elements. Only the head or nuclear portion of the sperm is of real 

 importance in the essential act of fertilisation ; the nutritive tail or cap 

 simply dissolves away. After the sperm-nucleus has penetrated^ to tlie 

 centre of the ovum, and after the extrusion of the polar bodies is quite 

 completed, we have to deal with two nuclei, not only closely approximate 

 in structure, but alike in further history. 



In Carnoy's type, both male and female nuclei contain two chromatin 

 elements, in the form of bent rods; and before union takes place, these 



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