LECTURE XV 

 THE PROCESS OF FERTILIZATION 



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

 inheritance — The role 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-cell — Beducing division in parthenogenetic eggs — In the bee — Exceptional 

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

 parthenogenesis. 



Now that we have made ourselves acquainted with the two kinds 

 of 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,-4) 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 (G). In certain worn-out or 

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

 occur— the so-called ' 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 



