472 



The nutritive cells, on the other hand, gradually diminish 

 in size, and are left as a little clump of disappearing cells in 

 close contact with the ovum. 



In the female, but not in the male, is a pair of glands 

 (fig. 188) lying in close contact with the anterior extremity 

 of the ovarian tubules. They consist " of large cells with 

 granular cytoplasm, and open on to the abdomen on either 

 side just behind the petiole. The glands themselves contain 

 a distinct cavity. I have not observed their mode of develop- 

 ment, but they seem to be formed simply as a depression in 

 the ectoderm early in pupal life. 



What the function of, the glands is, is difficult to deter- 

 mine. Their occurrence in the female alone indicates that 

 they are sexual excitants of some kind. 



THE NERVOUS SYSTEM. 



As early as 1832 Newport, comparing the simple type 

 of nervous system of the larva of Sphinx ligustri with the 

 more specialized condition ^ with its concentration of ganglia, 

 that he observed in the adult moth, showed that during 

 metamorphosis a "migration" of ganglia must occur- and 

 examining the pupa at various stages of development, he was 

 able to observe various intermediate conditions between tlie 

 larval and imaginal structures. 



But the first histological observations were made by 

 Weismann in 1864. He showed in the muscids that a process 

 of histolysis was going on within the ventral nerve cord ; the 

 nerve cells become dark and granular, while the whole nerve 

 cord changes into a structure of very fragile consistency. The 

 peripheral nerves become very pale, and losing their fibrillated 

 appearance, develop fine ref ractile globules in their interior. 



In Coretlira, on the other hand, a much simpler process 

 occurs; the central nervous system undergoes no fundamental 

 changes, and only where new organs develop are new peri- 

 pheral nerves formed. 



In 1889 Van Kees investigated the nervous system in 

 Calliphora, but could not confirm Weismann's observation 

 on the fatty degeneration of the peripheral nerves. So far as 

 I am aware, however, the cellular changes in the nerve cord 

 and peripheral nerves have never been investigated, and even 

 the work of Weismann does not contain any direct observa- 

 tions on the fundamental cell changes going on here during 

 metamorphosis. 



Tlie metamorphosis of the brain has received considerable 

 attention from Viallanes (1882, 1884, 1885), and much more 

 recently from Bauer (1904). I shall refer to the work of 

 these observers below. 



