256 THE EMBRYOLOGY OF THE BLOW-FLY IN THE EGG. 



rupture during development is very old. Weismann conceived 

 that in most Insects the development of the primitive band is 

 accompanied by a rupture of the blastodermic vesicle. He 

 divided Insects into two groups, one in which he believed such 

 splitting occurs, which he named regmagene Inseden ; others 

 in which he believed no rupture to occur he styled aregnia- 

 gene. 



According to the same authority, Chironomus, Simulia, 

 Pulex, Donacia, and Phryganea are ' regmagene,' whilst 

 Musca and Melophagus are * aregmagene.' 



E. Metschinikoff,* in 1886, said on this point : ' Although it 

 must be admitted that the primitive band has not the same rela- 

 tion to the remainder of the blastoderm in different insects, 

 it must not be concluded that it is differently developed since 

 no tearing of the blastoderm occurs. The variation is due to 

 the complete or incomplete character of the amnion.' 



In Weismann's * aregmagener Insecten ' no amnion is de- 

 veloped, and, as I have already stated, an amnion is developed 

 in the fly embryo. If I am correct in this, unless the Pupi- 

 parse are without an amnion, Weismann's second class, re- 

 garded as insects in which the amnion is not formed, does not 

 exist. 



According to my observations, the development of the fly 

 embryo is very similar to that of Chironomus, as given by 

 Weismann, and the apparent rupture of the serosa described 

 by him in the latter is merely the result of the shortening of 

 the primitive band and the separation of the head and tail 

 folds. 



I confess that the exact relation of the dorsal plate to the 

 amnion has greatly puzzled me, although, if Graber's figures 

 are correct (PI. XIII.), it is clearly first formed before the 

 serosa exists, and, if my interpretation of them is right, when 

 the amnion is a thick-walled epithelial tube ; and is only after- 

 wards covered by the extension of the amnion and serosa over 

 the dorsal surface of the embryo. 



It is important to remark that both the dorsal involution of 

 « L.c, p. 254. 



