The Egg 



157 



of liis first paper AVeismami liad not ascertained the 

 destination of tlie ' polar globules ' of Robin, and tliouglit 

 that they Avere subseqnently cast out from the embryo, 

 at least in part. After the appearance of Metschnikoff's 

 account of the development of Miastor, and Balbiani's 

 account of the development of Chironomus, AVeismann 



adopted their view that the 

 so-called j)olar globules are 

 sexual germs. Balbiani 



(1885) contributed some new 

 and interesting particulars, 

 and traced the development 

 of the reproductive organs 

 from the so-called polar glo- 

 bules. Eitter (1890) was the 

 first to apply the method of 

 sections to the eggs of Chiro- 

 nomus. He gave the first 

 satisfactory account of the 

 origin of the layers of the 

 alimentary canal, and fur- 

 nished needful corrections as 

 to the process of egg-laying. 



The egg is of elongate -oval The egg. 



fpn, female ditto, pg, polar cells or form, -Q mm. (-012 in.) long, 

 globiilos. pr, external protoplasmic ^ ^ ^ ' 



and -I mm. (-004 in.) broad. 



The anterior end, at which 

 the head of the larva Avill appear, is rather blunter than 

 the other, and one side is flattened. There is a trans- 

 parent and structureless egg-shell, which is perforated at 

 the anterior end by a minute micropyle for the entrance 

 of the spermatozoa. Within the egg-shell is a vitelliije 

 membrane, hardly to be seen in an undeveloped egg, but 

 becoming plain when the embryo shrinks, as it does in 

 the course of development. Almost the whole space 



Fig. 117. — Egg just laid, in longitu- 

 diual section, mpn^ male pronucleus 



layer, y, yolk. (From Ritter, i 

 fig- I.) 



