THE VITAL PROPERTIES OF THE CELL 265 



segments, in the nuclei of egg- and sperm-cells there are but 12. 

 It is only when the two nuclei unite that they form a complete 

 nucleus, from which arises the first division spindle with its 24 

 mother-segments, 12 being of male and 12 of female origin. 



As concerns the centrosomes, a slight difference is shown by 

 Echinoderms and Phanerogams. In the former, the centrosome 

 at the beginning is single in both egg- and sperm-nuclei, and 

 only becomes doubled through division ; in the latter, on the other 

 hand, two centrosomes are seen at a very early period both in the 

 pollen-tube and in the egg-cell. 



If we compare the results mentioned on the preceding pages 

 (256-264), we may lay down the following fundamental laws re- 

 ferring to the process of fertilisation as it occurs in animals and 

 phanerogamous plants : 



During fertilisation morphological processes, plainly to be 

 demonstrated, occur. The most important and essential of these 

 is the coalescence of the two nuclei which are derived from differ- 

 ent sexual cells, that is to say, the coalescence of the egg- and the 

 sperm-nuclei. 



During the act of fertilisation two important processes of coa- 

 lescence occur : 



1. Equivalent quantities of male and female stainable nuclear 

 substance (nuclein) unite together. 



2. Each of the halves obtained by the division of a male centro- 

 some unites with a corresponding half of a female centrosome, 

 by means of which the two centrosomes of the first nuclear 

 division figure are obtained. 



In the male and female alike, the stainable nuclear substance 

 has been reduced to one half of the normal quantity, both as re- 

 gards mass and the number of nuclear segments which it contains. 

 Hence it is only after they have fused together that the full 

 amount of substance and the complete number of segments con- 

 tained by a normal nucleus are again present. 



3. The Fertilisation of Infusoria. Certain Infusoria afford us 

 especially important objects for the investigation of the subject 

 of fertilisation. The sexual processes occurring in them w r ere 

 discovered by Balbiani and Biitschli (VII. 6), who were pioneers 

 in this work, and they have lately been rendered much clearer by 

 the classical labours of Maupas (VII. 30) and of Richard Hertwig 

 (VII. 21). 



Infusoria, as it is well known, differ from other lower organisms 



