625 



the ninth sternal region. These invaginations are completely distinct 

 from one another. Similar pairs of invaginations may be found in the 

 fully grown larvae of Pontia brassicae and Pygaera hucephala. 



2) During the quiescent period preceding pupation changes take 

 place. The anterior vesicles enlarge, lose their paired character and 

 become elongated antero-posteriorly. A tube invaginated from the 

 hypodermis connects them to the ventral ends of the larval oviducts. 

 They are afterwards connected by a furrow converted later on into a 

 tube to the second pair of vesicles. This furrow and tube are also 

 derived from the hypodermis. The posterior vesicles retain their paired 

 character and each of them begins to develop a posterior tubular out- 

 growth. An aperture corresponds to the base of the first pair of vesi- 

 cles, and another aperture to the base of the second pair. 



3) During pupal life the structures assume by degrees the form of 

 the organs of the imago. The anterior end of the first vesicle becomes 

 the bursa copulatrix, the posterior end the receptaculum seminis. These 

 two structures are at first perched on the dorsal aspect of the azygos 

 oviduct, and are closely connected at their point of origin. At a later 

 period the bursa copulatrix is placed laterally to the azygos oviduct, 

 and finally the connection between the two is converted into a tube — 

 the seminal canal : but the bursa, and not the oviduct, retains a con- 

 nection with the original external aperture of the first pair of vesicles. 

 The receptaculum seminis maintains its position. The two posterior 

 vesicles are converted into the paired «sebaceous« glands. They open 

 into the azygos oviduct by a common duct, and their tubular processes 

 grow to an immense length. The azygos oviduct or vagina which is 

 derived from the tube connecting the first pair of vesicles to the larval 

 oviducts, and from the tube connecting them to the posterior pair of 

 vesicles grows in length. Its posterior aperture shifts backwards until 

 it lies immediately ventral to the anus and some distance behind the 

 second pair of vesicles. It acquires a very distinct chitinoid lining at 

 an early period. A pair of glands — odoriferous glands of von Sie- 

 b o 1 d — absent in Pontia brassicae develop as invaginations from the 

 hypodermis immediately beneath i. e. in front of the oviducal aperture. 

 Finally a circular fold of hypodermis grows inwards and surrounds the 

 anus, the oviducal aperture, and the aperture of the odoriferous glands. 

 1 he area inclosed by the fold corresponds to a portion only of the tenth 

 somite and constitutes the terminal papilla of the abdomen. The epi- 

 thelium of the azygos oviduct and the accessory organs is derived solely 

 from the hypodermis. 



Two results follow from the above-given description. First, that 

 the aperture of the bursa copulatrix belongs to the eighth somite and 



