THE VITAL PROPERTIES OF THE CELL 299 



After the separation of the first polar cell, that half of the 

 spindle, which was left behind in the ovum, develops into a com- 

 plete spindle again, just as if the second polar cell were going 

 to be divided off . However, this does not occur; for the second 

 spindle only divides into two nuclei, which remain in the ovum 

 itself. After some time they fuse together in this place, and 

 drifting towards the middle of the yolk, again produce a single 

 nucleus as it were by self-fertilisation ; by means of this nucleus 

 the parthenogenetic processes, which quickly follow, are introduced. 

 Thus, in this case, the second division, the purpose of which is to 

 reduce the nuclear mass and to prepare for subsequent fertilisation, 

 is abortive. That by this means no sufficient compensation is made 

 for the absence of fertilisation is evident from the subsequent 

 course of the parthenogenetic process of fertilisation, i.e. from the 

 more or less premature death of the ovum. 



From the circumstance, that in parthenogenetic development 

 the formation of the second polar cell does not occur or is abortive, 

 we might conclude, that development invariably becomes im- 

 possible after the nuclear mass has been reduced to one half of 

 its normal amount, unless a fresh stimulus is given to the organism 

 by means of fertilisation. However, at present, this conclusion, 

 which perhaps contains some truth, cannot be said to be generally 

 applicable. For Platner (VII. 47), Blochmann (VII. 46), and 

 Henking (VII. 17) have observed, that the ova of certain 

 Arthropods (JLiparis dispar, Bees) develop in a parthenogenetic 

 manner into normal animals, although, like ova which require fer- 

 tilisation, they have produced two polar cells. In these cases a 

 more careful investigation of the circumstances with reference to 

 the number of the nuclear segments is certainly desirable. 



Hence, at any rate, it must be admitted, that it is possible for 

 ova, which contain reduced nuclei as a result of the formation of 

 the two polar cells, to develop further in a parthenogenetic 

 manner ; for nuclei, which contain a reduced amount of nuclein, 

 have in no way lost their capacity for division, as may be easily 

 supposed. An experiment, conducted by Richard Hertwig and 

 myself (VI. 38, 32), upon the ova of the sea-urchin, proves this in 

 a striking manner. 



By shaking the ova of sea-urchins violently, they can be split up 

 into small portions, which do not contain nuclei ; these then be- 

 come globular, and exhibit signs of life for a fairly long time ; 

 further they may be fertilised by spermatozoa. By this means 



