702 THE CEXERATll'E ORGANS. 



which has been very generally adopted in text-books and widely 

 accepted, is that the egg-cell only is enclosed in the chorion, 

 and that the nutrient cells remain outside the primitive chorion 

 in contact with the micropyle, and ultimately disappear or are 

 absorbed in the nutrition of the egg-cell. 



This view is very tempting on theoretical grounds ; and if it 

 were true many difficulties in the interpretation of the nature 

 of the Meroistic ova of Insects would disappear. The Meroistic 

 and Panoistic ova would then only differ in the manner in which 

 the egg-cell is nourished, and the yelks would represent single 

 cells, the nuclei of which might be regarded as germinal vesicles. 



A careful examination of the ova in all stages of development 

 has, however, convinced me that there are no grounds in the 

 Blow-fly at least for Brandt's view. Weismann [2] states dis- 

 tinctly that the nutrient cells are enclosed within the chorion 

 with the egg-cell, and that they all take part in the formation 

 of the yelk by fusing into a single mass. 



It is a fact of great significance in this relation that the 

 nutrient cells do not shrivel and disappear, but they increase in 

 size like the egg-cell, only more slowly, and they continue to 

 increase in size after the egg-cell is entirely converted into yelk 

 and in turn become converted into yelk themselves. 



Numerous drawings have been published, notably by 

 Korschelt [349, Fig. 49], and Henking [350], in which the 

 .chorion is represented between a group of cells and the mature, 

 .or nearly mature, ovum. These puzzled me for a long time, 

 until I discovered a similar appearance in one of my own 

 sections. Since then I have several times seen the same 

 thing, and it is clearly due to the displacement of an immature 

 ovum, which, being soft, has formed a kind of cap over the 

 anterior end of a mature ovum behind it. 



The so-called alternating yelk bodies and germ ova, which 

 are seen so frequently in the ovarian tubules of many insects 

 in which numerous ova arc developed in each tubule, are more 

 difficult to explain ; but it appears to me probable that in such 

 cases several series of ova are developing at the same time ; 

 and that the group of cells described as intervening between 



