78 FUNCTION OF POLAR CELLS. 



as we probably must, that there are genuine cases of such parthenogenesis, 

 it cannot be taken for granted without more extended observation that the 

 occurrence of development in these rare instances may not be due to the 

 polar cells not having been formed as usual, and that when the polar cells 

 are formed the development without impregnation is impossible. 



Selenka found in the case of Purpura lapillus that no polar body was 

 formed in the eggs which did not develop, but in the case of Neritina, 

 Biitschli has found that this does not hold good. 



The remarkable observations of Greeff (No. 88) on the parthenogenetic 

 development of the eggs of Asterias rubens tell, however, very strongly 

 against the above hypothesis. Greeff has found that under normal 

 circumstances the eggs of this species of starfish will develop without 

 impregnation in simple sea-water. The development is quite regular and 

 normal, though much slower than in the case of impregnated eggs. It is 

 not definitely stated that polar cells are formed, but there can be no doubt 

 that this is implied. Greeff's account is so precise and circumstantial that 

 it is not easy to believe that any error can have crept in ; but neither 

 Hertvvig nor Fol have been able to repeat his experiments, and we may be 

 permitted to wait for further confirmation before absolutely accepting them. 



To the suggestion already made with reference to the function of the 

 polar cells, I will venture to add the further one, that the function of 

 forming polar cells has been acquired by the ovum for the express purpose of 

 preventing parthenogenesis. 



The explanation given by Mr Darwin of the evil effects of self-fertiliza- 

 tion, viz. the want of sufficient differentiation in the sexual elements 1 , 

 would apply with far greater force to cases of parthenogenesis. 



In the production of fresh individuals, two circumstances are obviously 

 favourable to the species, (i) That the maximum number possible of fresh 

 individuals should be produced, (2) That the individuals should be as 

 vigorous as possible. Sexual differentiation (even in hermaphrodites) 

 is clearly very inimical to the production of the maximum number of 

 individuals. There can be little doubt that the ovum is potentially capable 

 of developing by itself into a fresh individual, and therefore, unless the 

 absence of sexual differentiation was very injurious to the vigour of the 

 progeny, parthenogenesis would most certainly be a very constant occur- 

 rence ; and, on the analogy of the arrangements in plants to prevent self- 

 fertilization, we might expect to find some contrivance both in animals and in 



open end of the uninjured oviduct ; and though Oellacher's instances are more valuable, 

 yet sufficient care seems hardly to have been taken, especially when it is not certain 

 for what length of time spermatozoa may be able to live in the oviduct. For Oellacher's 

 precautions, vide Zeit. fur Wiss. Zool., Bd. xxii., p. 202. A better instance is that 

 of a sow given by Bischoff, Ann. Sci. Nat., series 3, Vol. ii., 1844. The unimpreg- 

 nated eggs were found divided into segments, but the segments did not contain the 

 usual nucleus, and were perhaps nothing else than the parts of an ovum in a state of 

 disruption. 



1 Darwin, Cross- and Self - Fertilization of Plants, p. 443. 



