16 



FEKTILISATION 



It expands, however, before conjugation takes place, and the two nuclei, now 

 of equal size, lie side by side. In each the nuclear network is converted into a 

 spireme thread, the membrane disappears, and the thread is divided into the 

 chromosomes. The paternal and maternal chromosomes thus form separate 

 groups, and these appear to remain distinct as the rods from each nucleus are 

 gathered into the equatorial plate of the first segmentation-spindle. The mixture 

 of the chromatin is probably not effected, therefore, until the first segmentation- 



FIG. 25 (1-7). STAGES IN THE FERTILISATION OF THE EGG OF THE MOUSE. (Sobotta.) 



1, Entrance of spermatozoon ; 2, rotation of sperm-head ; 3, formation of sperm-nucleus, which 

 lies to the left ; the germ-nucleus lies to the right ; 4, resolution of nuclei ; 5, vesicular stage of nuclei ; 

 the smaller is the sperm-nucleus ; 6, enlargement of sperm-nucleus and its approach to germ-nucleus ; 

 7, first segmentation-spindle, with group of paternal chromosomes to left and of maternal to right. 



division has begun ; and it has been proved in Ascaris, which has only four 

 chromosomes, that the rods are divided, after splitting, between the two first blasto- 

 meres in such a way that each receives an equal number of paternal 

 and maternal chromosomes (fig. 27). This is a fact of capital importance, 

 and plays a prominent part in modern theories of heredity.- 



If we analyse the phenomena of fertilisation we must recognise two factors : (1) The initiation 

 of cell-division, and (2) the union of the nuclei. Though closely bound up with one another, 

 these two factors are distinct and independent phenomena. Thus it has been demonstrated 



