538 MATURATION AND IMPREGNATION OF THE OVUM. 



should it be confirmed, may perhaps be explained on the 

 hypothesis, I have just suggested, viz. that a more or less essential 

 part of the nucleus is removed in the formation of tJie polar cells ; 

 so that in cases, e.g. Arthropoda and Rotifera, wJiere polar cells are 

 not formed, and an essential part of the nucleus not therefore 

 removed, parthenogenesis can much more easily occur titan when 

 polar cells are formed. 



That the part removed in the formation of the polar cells is 

 not absolutely essential, seems at first sight to follow from the 

 fact of parthenogenesis being possible in instances where impreg- 

 nation is the normal occurrence. The genuineness of all 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 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 

 less possible. 



The remarkable observations of Professor Greeff (19) on the 

 parthenogenetic development of the eggs of Asterias rubens tell, 

 however, very strongly against this explanation. Greefif 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. Professor Greeffs 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 per- 

 mitted to wait for further confirmation before absolutely accepting 

 them. 



1 The instances quoted by Siebold from Hensen and Oellacher are not quite 

 satisfactory. In Hensen's case 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. fiir iviss. Zool, Bd. xxn. p. 202. 



