8 H E WITT, Cytological A sped of Parthenogenesis in Insects. 



CHALCIDID/E. 



The embryonic development of some species of 

 parasitic Hymenoptera is very remarkable and extremely 

 interesting. As the unfertilised eggs of the species 

 investigated are able to develop parthenogenetically 

 giving rise to males, they will be considered here. 



Encyrtus fuscicollis. This small hymenopteron 

 deposits its eggs in the eggs of the larva of a lepidopteron 

 Hypono7ne2ita cognatella, and probably in other cater- 

 pillars. Bugnion (19) first described the development of 

 the larval Encyrtus but did not study the embryological 

 changes. This, however, has been done recently by 

 Marchal (60) whose account is most clear, and is accom- 

 panied by excellent figures. The first stage of the ovum 

 which he was able to obtain was that in which there were 

 two nuclei. No mention is made of polar bodies as he 

 was unable to obtain earlier stages. The ovum lies in the 

 body cavity of the embryonic caterpillar, and grows rapidly. 

 One of the nuclei, which he calls the paranucleus, grows 

 very large, the others subdividing until a large number 

 are formed. These last collect in groups, the protoplasm 

 surrounding them becomes rounded off and they form 

 separate morulae. The paranucleus divides up into a large 

 number of parts, which ultimately with their accompany- 

 ing cytoplasm form an investing sheath round the morulae. 

 External to this an enclosing membrane is formed from 

 the tissues of the host. By continued growth and 

 development of the morulae, they displace one another 

 until they are placed end to end in a string. Each of 

 these morulas forms a separate embryo. They are all 

 enclosed by two investing membranes, an inner which 

 Marchal terms the trophoamnion, which also separates 

 the embryos from each other by transverse septa, and is 



