LECTURE XV 

 THE PEOCESS OF FERTILIZATION 



Cell-division and nuoleai' division — The chromatin as the material basis of 

 inheritance — The rOIe of the centrosphere in the mechanism of division — The 

 Chromosomes— Fertilization of the egg of the sea-urchin according to Hertwig — Of 

 the egg of Ascaris according to Van Beneden — The directive divisions, or the extrusion 

 of the polar bodies — Halving of the number of chromosomes — The same in the 

 sperm-oell — Reducing division in partheuogenetic eggs — In the bee — Exceptional 

 and artificial parthenogenesis — RSle of the centrosphere in fertilization and in 

 parthenogenesis. 



Now that we have made ourselves acquainted with the two kinds 

 oi' germ-cells on the union of which ' sexual reproduction ' depends, we 

 may proceed to a more detailed discussion of the process of fertilization 

 itself. But it is indispensable that we should take account of the 

 processes of nuclear and cell-division, as these have been gradually- 

 recognized and understood in the course of the last decade. It may 

 appear strange that the processes of division should throw light on the 

 apparently opposite processes of cell-union, but it is the case, and no 

 understanding of the latter is possible without a knowledge of the 

 former. 



From the time of the discovery of the cell until well on in the 

 sixties the process of cell-division was looked on as a perfectly simple 

 process, as a mere constriction in the middle of the cell. It was 

 observed that a cell in the act of dividing (Fig. 59,^) stretched itself 

 out, that its nucleus also became longer, became thinner in the middle, 

 assumed a dumb-bell form, and was then gradually constricted, giving 

 rise to two nuclei (B), whereupon the body of the cell also constricted 

 and the two daughter-cells were formed (C). In certain worn-out or 

 highly differentiated cells a cell- division of this kind really seems to 

 occur— the so-caUed ' direct' division — but in young cells, and indeed 

 in all vigorous cells, the process, which looks simple, is, in reality, 

 exceedingly complex. Not only is the structure of the nucleus incom- 

 parably more complex than was recognized a quarter of a century ago, 

 but nature has placed within the cell a special and marvellously 

 intricate apparatus, by means of which the component parts of the 

 nucleus are divided between the two daughter-nuclei. 



For a long time all that was distinguished in the cell-nucleus was 



