238 THE EMBRYOLOGY OF THE BLOW- FLY IN THE EGG. 



production of the blastoderm, except that it serves to nourish 

 the growing morula formed by the segmenting germ, but which, 

 nevertheless, is directly concerned in the development of the 

 parablast in other words, which may be regarded as a part of 

 the mother organism, by which the germ is nourished. 



Theoretically, according to this view, the parablast might be 

 regarded as of parthenogenetic origin ; but it must be remem- 

 bered that the parablastic cells would probably be affected by 

 the growth of the morula, and the second or third generation 

 of amoeboid parablastic elements may possibly become more or 

 less modified by the male parent. That parthenogenesis exists 

 in insects is well known, and it may be that the origin of the 

 parablast directly from the mot her is a relic of true partheno- 

 genesis. 



The Polar Cells of Weismann. In the Diptera, more especially 

 in Chironomus, the polar cells of Weismann actually exhibit the 

 same segmentation phenomena as the holoblastic ovum, and I 

 believe they represent the vegetative pole of the morula, which 

 results from the segmentation of the germ-yelk. Weismann 

 [2] described these cells in the egg of Chironomus, which is 

 exceedingly favourable for their observation. He says : ' In the 

 youngest eggs I have observed that the yelk is covered by a 

 highly-refractive bluish layer of germinal blastema (Keimhaut- 

 blastem). At the hinder pole of the egg, in a space between the 

 germinal blastema and the yelk-sac, are four large oval or 

 spheroidal cells, which I term polar cells.' 



I would remark that my sections show that the layer of clear 

 yelk substance, which is believed by some to become the blasto- 

 derm, exists in the unimpregnated ovarian yelks. It probably 

 represents the white yelk of Birds and Reptiles. 



Weismann observed that the polar cells undergo binary 

 division, so that the four cells become 8, 16, and 32 successively. 

 With regard to the polar cells of the Blow-fly embryo, he 

 observes that, ' although easily overlooked, they conform in 

 their manner of multiplication with those of Chironomus.' I 

 have been unable to detect them until they form a plate of 

 relatively small cells at the posterior pole of the blastodermic 



