l8o THE EVOLUTION OF SEX. 



is unknown {e.g., leeches and Sagitta), also exhibit the same 

 early differentiation of germ-cells, so that we can only look 

 upon the fact as one of the auxiliaries of parthenogenesis. 



(c) The peculiarity of parthenogenetic ova, which has of late 

 attracted much attention, is that they extrude only one polar 

 cell, — not two, like other eggs. This discovery is due to Weis- 

 mann, who, with the assistance of Herr Ischikawa, has verified it 

 in about a dozen species, Leptodora hyalifia, Sida crystallina, 

 Cypris reptans., and other water-fleas. Blochmann has also 

 corroborated Weismann's discovery, in his observations on 

 aphides. What theoretical importance Weismann attaches to 

 the fact will be immediately noticed."^ 



§ 8. Theory ofFarthenogefiesis. — We may begin with Balfour's 

 view of the case, though that of Minot has the priority. " The 

 function of forming polar cells has been acquired by the ovum 

 for the express purpose of preventing parthenogenesis." If they 

 were not formed, parthenogenesis would normally occur. This 

 is expressed in curiously teleological language, but the main 

 idea is clear enough, — the retained polar cells replace the sperm 

 nucleus. It is only necessary to change cells into cell to make 

 it reasonable to-day. One must not forget, however, that in 

 higher animals, where parthenogenesis is unknown, polar cells 

 have not been found often as yet, nor ever seen in birds and 

 reptiles. And one would fain get further back still, and know 

 7vhy only one polar globule is formed in parthenogenetic ova. 



" In accordance with Minot's hypothesis of sexuality, it 

 might be assumed that in parthenogenetic ova the male element 

 was retained, and that the cell remained a true asexual cell, and 

 did not become a sexual element." " Blochmann and Weis- 

 mann have shown that this is the case, by their discovery that 

 in parthenogenetic ova only one polar globule is formed, while 

 there are always two in ova which are impregnated ; hence it is 

 probable that one polar globule (by hypothesis, male) is re- 

 tained." 



Minot's words are not beyond criticism either, though they 

 are not teleological. An ovum which retains a male element 

 is not happily described as remaining asexual ; it would be better 

 to call it a case of intra-cellular hermaphroditism. Nor can it 

 yet be said that there are always two polar globules in ova 



* Blochmann, however, claims to have demonstrated the formation of 

 tzvo polar bodies in those unfertilised eggs which are to give birth to drones. 



