432 THE ALLMEXTARY CAA'AL OF THE IMAGO. 



that the convoluted gland tubes are developed, and they remain 

 connected \vith the oesophagus, as Weismann represented them 

 [2, PI. IX., Fig. 15], for a long time. 



Hence, I think it is probable that the new gland tubes are 

 developed from cells formed by proliferation from the blind end 

 of the duct, just as the gland tubules originate in the mammary, 

 salivary and lachrymal glands of Vertebrates, as solid cell growths 

 from the Malpighian layer of the epidermis. The imaginations 

 which form the distal extremities of the ducts either appear 

 at a subsequent stage or are comparatively short when the 

 gland is first formed ; the latter condition has apparently led to 

 the received view with regard to the manner in which the 

 tubular glands of insects are developed from the epiblast. 



Addendum. \Yhen the above was already in type, my atten- 

 tion was drawn to a very important paper by Paul Mayer* on 

 the development of some decapod Crustacea. Mayer describes 

 the development of the hind-gut, as Graber did from the blasto- 

 poral imagination. In this he completely agrees with my views, 

 except that he describes no proctodaeum in the sense in which 

 I have used the term. Great confusion has arisen by the use 

 of the words hypoblast and mid-gut with meanings which are 

 special to w^orks on insect embryology. If the mid-gut is ever 

 developed from free yolk cells in Arthropods, hypoblast is not 

 an appropriate designation for the layer of cells which forms 

 its wall ; nor can the term proctodaeum be properly applied to 

 a blastoporal involution. The application of the latter some- 

 times to one and sometimes to another involution has led to 

 erroneous generalisations. 



k Mayer, Paul, 'Zur Entwickelungsgeschichte der Dekapoden,' Jenaische 

 Zcitsch. f. Naturwissenschaft, Hd. xi., 1877. 



