THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 23 



from the nucleus of the ooblast to form ovarian epithelium and nutritive cells ; on 

 the contrary, the nuclei after leaving the end chamber are arranged in circular 

 masses, the outer nuclei of which are smaller than the enclosed nuclei. The latter, 

 in more advanced egg chambers, can be recognized as the nutritive cells, while one 

 differing from the nutritive cells in appearance becomes the germinal vesicle. 



The germinal vesicle was not observed in the younger egg chambers, that is, 

 those nearest the end chamber of the tube. Each chamber of the tube with the ex- 

 ception of the end chamber forms a single mature egg. 



Figure LXVII represents the last chamber of the egg tube in a more advanced 

 stage than that represented by CBIV Fig. LXVI. 



The ovarian epithelium (OVE in Fig ) has become quite columnar at the end 

 of the egg chamber towards the outer end of the ovarian tube. The nutritive cells 

 at this end of the chamber have broken down, forming yolk, (Y in Fig. LXVII). 

 The yolk consists of small spherules, in every respect similar to spherules found 

 in the nuclei of the nutritive cells (N, C in Fig.) The yolk of the mature fly's egg 

 is of a similar character. In all probability then the yolk consists of the broken 

 down nuclei of the nutritive cells. In the mature ovum it is surrounded by a proto- 

 plasmic sheath which is probably derived from the protoplasm separating the nuclei 

 of the nutritive cells. 



Figure LXVIII represents a longitudinal section of a more advanced stage in 

 which the germinal vesicle is seen lying in the yolk. It consists of finely granular 

 protoplasm which, unlike the nuclei of the nutritive cells, does not stain. At this 

 stage, the vesicle has a definite boundary. It is eccentric in position, lying near the 

 ovarian epithelium, which at this stage, has begun to excrete the chorion (CH in 

 Fig.) on its inner surface. The ovarian epithelium apparently takes no part in the 

 formation of the yolk. After the formation of the chorion it adheres as flattened 

 hexagonal cells to the outer surface of the latter even when the egg is laid. 



Figure LXIX is a drawing of a cross section through an egg chamber at the 

 same stage of development as that represented in figure LXVIJI. In this figure the 

 deeply stained germinal spot is represented in the centre of the germinal vesicle 

 (GS in Fig). 



The germinal spot is not homogeneous throughout, but has in its centre what 



