^894. . CELL-DIVISION. 421 



has observed that, in one species of Ascavis, both the egg- and sperm- 

 nuclei (male and female pronuclei), contain a single varicose chro- 

 matin thread. As they approach one another, the thread in each 

 divides transversely into two halves, both of which become looped and 

 •again divide, this time by longitudinal splitting, into four apparently 

 identical pieces. The two nuclei then come into close contact, and 

 the eight threads, from male and from female, arrange themselves in 

 •the equatorial plane of a spindle, which has by this time been formed 

 'by the activity of the centrosome brought into the ovum by the 

 ■spermatozoon. The threads gradually travel towards the poles of 

 the spindle, and a separation is effected between them in such a way 

 that each of the daughter-nuclei is formed out of four chromo- 

 somes, two derived from the female and two from the male. Con- 

 strictions now appear, dividing the ovum into two blastomeres, the 

 nuclei of which have by this time passed into a resting condition. 

 It will be seen from the foregoing account that there is no true 

 fusion of the chromosomes of the two pronuclei, but only a mingling 

 together of their substance. Indeed, the notion that in the fertilisa- 

 tion of any ovum the two nuclei actually fuse to form one, or that 

 any part of them do so, is gradually being given up. 



There still remain to be considered those cases in which the two 

 nuclei copulate while in a typically resting state. The process of 

 fertilisation in the leech Nephelis, which the present writer has had 

 the opportunity of studying, affords a good case in point. Both pro- 

 nuclei are in a state of perfect rest during the whole time ; all that 

 appears to happen is the breaking down of the two membranes of the 

 adjacent nuclei at the point of contact, allowing a mingling of the 

 chromatic and achromatic elements, which results in the formation 

 of a single large nucleus. This also remains some time in a state of 

 rest before it proceeds to karyokinetic division. 



It would seem, therefore, necessary to distinguish those cases in 

 which the two pronuclei fuse, so as to give rise immediately to a 

 nuclear figure, from those in which copulation takes place between 

 two resting pronuclei to form a single nucleus in the same condition. 

 What the significance of these two modifications may be is not clearly 

 understood. 



Nuclear Division in Relation to Points of Theoretical Importance. 



Since the nuclear matter is so obviously concerned in the 

 process of fertilisation, it is legitimate to hold that the nucleus is the 

 seat of the hereditary substance which the parent transmits to its 

 offspring. Strasburger and O. Hertwig were among the first to 

 advocate this view, and it has now come to be generally accepted 

 that only the chromatin of the nucleus is the bearer of the hereditary 

 tendencies. This view is supported by the fact that paternal 

 characters can be inherited just as well as maternal, although the 

 amount of protoplasm in the two germ-cells is greatly disproportionate. 



