THE NYMPHOID STAGE OF THE EMBRYO. 257 



the dorsal plate and the proctodaeum exist at the same time in 

 Hydrophilus (Graber) and in Blatta (Cholodkowsky), as this is 

 strong confirmatory evidence in favour of my statement that 

 the proctodaium and dorsal involution are distinct in the fly 

 embryo. Moreover, it disposes of Graber's contention that 

 the dorsal invagination — my archenteron — is a hind gut, and it 

 establishes the relation of the Malpighian tubules with the 

 mid-gut, and not with the proctodaeum (see Development of 

 Malpighian Tubules). 



5. THE NYMPHOID STAGE OF THE EMBRYO AND ITS CON- 

 VERSION INTO THE LARVA. 



The Nymphoid Stage of the Embryo. — About twelve hours after 

 mpregnation the embryo has assumed what I term the nym- 



FlG. 42.- 



-An embryo about twelve hours old in the 

 Weismann.) 



nymphoid stage. (After 



phoid stage; the procephalic lobes meet in the middle line 

 above, and the rudiments of the cephalic post-oral appendages 

 attain their maximum development. The head-capsule in front 

 of the mouth and between and below the procephalic lobes pro- 

 jects as a vesicular swelling — the fore-head. The remainder 

 of the embryo is segmented as in the larva. In this condition 

 it has a closer resemblance to the nymph than at any inter- 

 mediate period ; the thoracic appendages are, however, entirely 

 wanting, and the rudimentary cephalic appendages exhibit a 

 verj' generalized type. 



Formation of the Larva. — After the twelfth hour the retro- 

 gression of the developmental process becomes apparent, and 



