ANATOMY AND MORPHOLOGY OF INSECTS. 17 



embryo differs in no material points from that given by the late 

 Professor Balfour in his comparative embryology ; except in 

 relation to the origin of the alimentary canal, in which my 

 observations agree \vith those of Butschli. My own investi- 

 gations, however, enable me to add the following important 

 statements, which will be more fully discussed in another 

 chapter : 



The primitive hypoblast behind is, I believe, the dorsal plate 

 of Kowalevski.* It becomes concave above, forming a semi- 

 cylindrical groove, the edges of which ultimately unite from 

 before backwards. The cavity so enclosed remains continuous 

 with that of the archenteron, and forms that portion of the 

 intestine between the vessels of Malpighi and the rectum, which 

 is usually regarded as originating from the proctodasum. It 

 is perhaps a part of the mesenteron, but for the sake of clear- 

 ness I shall term it the metenteron. 



The metenteron is ultimately enclosed by the upward 

 growth of the somatopleure, the edges of which meet and 

 unite above it. 



The dorsal vessel appears between the somatopleure and 

 the metenteron ; and the Malpighian vessels are developed as 

 hollow processes of the hypoblast at the junction of the mes- 

 enteron and the metenteron. 



The relations of the blastoderm to the yelk are somewhat 

 different in birds and in insects, so far as the Diptera are 

 concerned at least. If all the food-yelk of the bird's-egg 

 were transferred from the under surface of the hypoblast 

 to the cavity between the hypoblast and epiblast, or primi- 

 tive coelom, the hypoblast would correspond to the dorsal 

 plate of the insect's egg, and the amnion of the bird would 

 have the same relation to the epiblast and hypoblast as that 

 of the insect. f 



Ecdysis. In all insects, as in arthropods generally, growth 



* Kowalevski, A., ' Embryologische Studien an Wurmen und Arthropoden.' 

 Mem. Acad. Sci., St. Petersb., Bd. xvi., 1871. 



t These views were first promulgated by me in my Hunterian lectures, 

 February, 1890. 



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