MATURATION OF THE OVUM. 63 



and that if polar cells were not formed parthenogenesis might 

 normally occur. A strong support for this hypothesis would be 

 afforded were it to be definitely established that a polar body is not 

 formed in the Arthropoda and Rotifera ; since the normal occurrence 

 of parthenogenesis is confined to these two groups. It is certainly a 

 remarkable coincidence that they are the only two groups in which 

 polar bodies have not so far been satisfactorily observed. 



It is perhaps possible that the part removed in. the formation of the 

 polar cells is not absolutely essential ; and this seems at first sight to follow 

 from the fact of parthenogenesis being possible iii instances where impreg- 

 nation is the normal occurrence. The genuineness of the observations 

 on this head is too long a subject to enter into here 1 , but after admitting, 

 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 whrn 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 develope, 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 develope 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 

 Hertwig 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 leen acquired by the ovum for the express jmrpose 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 2 , 

 would apply with far greater force to cases of parthenogenesis. 



In the production of fresh individuals, two circumstances are obviously 

 favourable to the species. (1) That the maximum number possible of fresh 

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



J The instances quoted by Siebolcl, Ptrrthenofjcnesis d. Artliropoden, are not quite 

 satisfactory. In Hensen's case, p. 234, impregnation would have been possible if we can 

 suppose the spermatozoa to be capable of passing into the body-cavity through the 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.filr Wiss. Zool., Bd. xxii., p. 202. A better instance is that of a 

 sow given by Bischoff, Ann. Sci. Nat. series 3, vol. n., 18-14. The unimpregnated 

 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. 



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



