XII.] CONJUGATION AND SEXUAL REPRODUCTION. 175 



bryogeny, or, on the other hand, will suffice to bring it to 

 completion. In an earher work I have endeavoured to render 

 this theoretically intelligible and must here refer to that at- 

 tempt \ But I should wish to add in this place that I have, 

 since then, convinced myself that the view which I urged is 

 correct. In conjunction with Dr. Ischikawa, I have examined 

 the eggs of many Lepidoptera as to the power of development 

 without fertilization : we observed that, as a matter of fact, 

 some eggs entered upon embryogeny, which was, however, 

 sooner or later arrested in most of them, only a very few 

 reaching the caterpillar stage. Out of about a hundred unfer- 

 tilized ova o^ Aglia tau, we obtained only a single fully developed 

 caterpillar, many eggs shrivelled after a few days, while others 

 remained plump : in most of the latter the yolk contained a 

 large number of blastoderm cells ; for a whole month these 

 eggs developed very slowly and irregularly ^, but they finally 

 shrivelled and decayed. The ova of one and the same female 

 vary in respect to their powers of parthenogenetic development, 

 and such individual differences cannot lie in the yolk, inasmuch 

 as this nutritive material is distributed in the same manner and 

 in equal amount in all eggs : they must rather be referred to 

 differences in the rate of growth of the germ-plasm ; at any 

 rate, I cannot imagine any other cause which might account for 

 them. 



But this conclusion does not carry the imphcation that par- 

 thenogenesis could not have arisen by the method which was 

 first indicated, viz. by the suppression of the second polar body. 

 Indeed, I am incHned to believe that regular parthenogenesis 

 has invariably arisen in this way ; for otherwise the absence of 

 the second polar body would not be so common, nor would it 

 be without exception. This method cannot however obtain in 

 facultative parthenogenesis, because that very egg which is 

 capable of parthenogenetic development must also remain 

 capable of fertilization. But this latter capability involves that 

 reduction of the germ-plasm which occurs by means of the 

 second polar division. In those cases in which parthenogenesis 

 became necessary, and at the same time the capacity for fcrtili- 



1 Continuity of Germ-plasm.' Jena, 1885, pp. 92 et seqq. Translated 

 as the fourth essay; see Vol. I. pp. 231 et seqq. 



2 The observations were not directed to the details of embryogeny. 



