THE PROCESS OF FERTILIZATION 293 



the phenomena of inheritance. We shall then recognize that the rod- 

 or loop-shaped chromosomes cannot be simple elements, but are 

 composed of linear series of 10, 20, or more globular single-chromo- 

 somes, each of which represents a particular kind of chromatin or 

 hereditary substance. If we consider this carefully, we shall see that 

 it would hardly be possible to think out a mode of nuclear division 

 which would so exactly and securely fulfil the purpose of conveying 

 these many kinds of chromatin to the two daughter-nuclei in like 

 proportions as does the mechanism of distribution actually brought 

 about by nature. The longitudinal splitting of the rods halves tli<- 

 chromosomes, and the spindle apparatus secures the proper distribution 

 of the halves between the two daughter-nuclei. 



So much, at least, is certain, that no such complicated mechanism 

 for ' mitotic ' division would have arisen if the very precise division 

 of a substance of the highest importance had not been concerned, and in 

 this conclusion lies the first hint of the interpretation of the chromatin 

 substance as the bearer of the hereditary qualities. 



We are now familiar with the cell-nucleus and the apparatus for 

 its division, and we are thus fully prepared to begin the study of 

 the phenomena of ' fertilization.' Here also the processes depend essen- 

 tially on the behaviour of the cell-nuclei, for even the first observations 

 made by O. Hertwig on the behaviour of the spermatozoon after it has 

 penetrated into the ovum led to the suggestion that the essential 

 fact is the union of two nuclei ; and numerous later, more and more 

 deeply penetrating researches have furnished abundant evidence that 

 the so-called ' fertilization ' is essentially a nuclear fusion. 



Let us begin with O. Hertwig's observations on the ovum of the 

 sea-urchin. Eggs of this animal, which have been taken out of the ovary 

 of the female, may easily be fertilized artificially by pouring over 

 them spermatic fluid taken from a male, and diluted with sea-water. 

 Before this is done only one nucleus can be observed in the ovum, but 

 shortly afterwards two nucleus-like structures of unequal size can be 

 seen within the ovum, and the smaller is surrounded by a circle of 

 rays. Hertwig rightly interpreted this smaller nucleus as the modi- 

 fied remains of the penetrating spermatozoon, which then slowly 

 approaches the nucleus of the egg, and ultimately fuses with it to 

 form a 'segmentation nucleus.' From this starts the so-called 

 'segmentation' of the ovum, that is, the series of repeated divisions 

 resulting in the formation of an ordered mass of cells, which by 

 continued division of cells builds up the embryo. 



Simple as this process of nuclear conjugation may seem, it was by 

 no means so easy to recognize, and several investigators, especially 



