THE FORMATION OF THE PUPA. 115 



repeatedly kept flies for the purpose of discovering it, I have 

 never succeeded in getting the ova developed in captivity, per- 

 haps from the want of a proper kind of food. However well fed, 

 the insects only laid by large stores of fat in the ductless 

 glands. 



Impregnation is usually effected immediately before the eggs 

 are laid, the spermatozoa being probably discharged from the 

 spermatic receptacles upon the upper extremity of the ovum, 

 and brought into immediate contact with the whole length of 

 the yolk through the medium of the micropyle and yolk canal. 

 Immediately after the discharge of sperm, or simultaneously 

 with it, a viscid fluid is poured upon the ovum probably from 

 the albumen glands, which renders it glossy and opaque. The 

 act must be rapidly performed, for although the ova are usually 

 extruded from tke ovipositor in quick succession, none escape 

 impregnation. 



-Soon after impregnation, a layer of rapidly- growing cells 

 appears in the place of the germinal disc, and along the inner 

 surface of the yolk canal ; this layer forms tke ventral portiom 

 of the integument of the larva. At the end of eight hours it 

 kas extended so as to aurroami the entire yolk. The whole 

 integument is then sketched out by this oellwkr layer, and ita 

 segments are divided by well-marked folds* 



At this period, or foen after, tfee alimentary canal appears 

 sketched out by a cellular membrane the inaer or mucous 

 layer of the cells which surround the yolk. At first ft 

 is straight and very capacious, i&clmding the whole yoik, 

 but it becomes gradually narrow and tortuous during the 

 formation of intermediate layers of cells, from which the other 

 internal organs are developed. When the larva is hatched, * 

 still contains the remains of the yolk. 



