22 THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 



arated by a median depression, which is formed where the first groove or primitive 

 furrow had previously occurred. 



The cells forming the floor of the depression separating the nerve strings take 

 part in the formation of the nervous substance. This does not agree with my obser- 

 vations on the origin of the nervous system in Thyridopteryx, where the median 

 groove did not disappear before the formation of the nervous system. The latter 

 did not appear first as a single string. The cells lining the median groove separa- 

 ting the nerve strings did not apparently form any of the nervous substance. 



DIPTERA. 



Before going to the embryology of spiders, it may be well to insert here some 

 observations made on the maturation of the egg in Musca (domestica?). 



Each ovarian tube of the fly's ovum is divided, at certain stages at least, into 

 five divisions, viz : the end chamber (EC Fig. LXVI,) and four following chambers, 

 the largest (CB, IV, Fig. LXVI.) at the end of the tube. 



The ovarian tube consists externally of a membraneo'us peritoneal sheath in 

 which nuclei are embedded (FE in Fig. LXVI.) 



The end chamber contains nuclei, the histological structure of which could not 

 be determined. 



Korschelt, from observations on Musca vomitaria, finds that the egg cells and 

 nutritive cells which form the contents of succeeding chambers arise from the larger 

 nuclei contained in the end chamber, while the smaller nuclei of the end chamber 

 assume a superficial position in succeeding chambers, enclose the germinal vesicle 

 and nutritive cells and become the nuclei of the ovarian epithelium. 



Will, (l1 in an elaborate article on the origin of the yolk in insects, claims 

 that the nuclei of the end chamber (the so-called ooblasts) give off portions of their 

 substance, which form in some cases the nuclei of the ovarian epithelium and in 

 others the nuclei of the nutritive cells. 



The remains of the nucleus of the ooblast then form the germinal vesicle and spot. 



From the incomplete observations made on the embryology of Musca, it seems 

 that Korschelt's view is the correct one. There does not appear to be any budding 



(i) Will. Zeit f. wiss Zool. 1885. 



