298 THE CELL 



should require fertilisation and others not ? Weismann (VII. 40), 

 Blochmann (VII. 44), Platner (VII. 47), and others, have made 

 the interesting discovery, that parthenogenetic ova, and those 

 requiring fertilisation, exhibit an important and fairly essential 

 difference in the matter of the formation of the polar cells (vide 

 p. 236) ; whilst in the case of the latter two polar cells are divided 

 off in the usual manner, in that of the former the development 

 of the second polar cell, and consequently also the reduction of 

 the nuclear substance, which is otherwise connected with this 

 process, do not occur. Hence the egg-nucleus of the summer egg, 

 of a Daphnia, for instance, possesses without fertilisation the 

 whole nuclein mass of a normal nucleus. 



However, this interesting behaviour by no means explains the 

 nature of parthenogenesis. For the summer egg has the ten- 

 dency to develop without fertilisation, before it begins to form 

 the polar cells, as is seen from the small amount of yolk it con- 

 tains, the different nature of its membranes, etc. Hence the 

 ovum does not become parthenogenetic because it does not form 

 the polar cell ; but, on the contrary, it does not form the polar 

 cell because it is already destined for parthenogenetic develop- 

 ment ; it does not develop it because, under these conditions, the 

 reduction of the nuclear mass, which presupposes subsequent 

 fertilisation, is unnecessary. 



Many peculiar phenomena connected with parthenogenesis 

 have been observed, the closer study of which will probably con- 

 tribute much to the explanation of this question. Such a 

 phenomenon, the importance of which cannot at present be esti- 

 mated, is the fact, that the preparatory process for fertilisation 

 can be retraced, even after the polar cell has been formed. 



In many animals, the ova, if they are not fertilised, commence to 

 develop parthenogenetically, at the normal time. Attempts are 

 made by the ova of many worms, of certain Arthropods and 

 Echinoderms, and even of some Vertebrates (birds) to begin to 

 segment in the absence of male elements, and eventually to form 

 germinal discs ; but at that point they come to a standstill in 

 their development and die off. Abnormal external circumstances 

 seem to favour the occurrence of such parthenogenetic phe- 

 nomena in individual instances, as, for example, in Aster acanthion. 

 The following remarkable occurrence has been observed by Boveri 

 in Nematodes and Pterotrachea, and by myself in Asteracanthion, 

 during the formation of the polar cells. 



