THE ALIMENTARY CANAL 91 



epithelium is said to be derived from the ectoderm is Miastor metraloas, 

 a cecidomyiid whose paedogenetic development has been described by 

 Kahle (1908). 



Kowalewsky (1886), Graber (1889), Escherich (1900), Noack (1901), 

 and others who have studied the development of the Muscoidea are 

 generally agreed that the mid-gut epithelium arises from an anterior 

 and a posterior mesenteron rudiment which are distinctly separated 

 from the middle plate and each of which gives rise to a pair of entoderm 

 ribbons that develop into the epithelial layer. The species concerned 

 are Musca (Calliphora) vomitoria and M. erythrocephala, Lucilia caesar, 

 L. sylvarum {illustris), and Phormia regina. Voeltzkow (1889), however, 

 on the basis of his study on C. vomitoria states that lateral proliferations 

 on the blind ends of the stomodaeum and proctodaeum that result in the 

 usual pair of ribbons from which the epithelium is formed are ectodermal. 



In the sheep tick (Melophagus ovinus), one of the Hippoboscidae, 

 Pratt (1900) found that the first indication of the mid-gut rudiment is a 

 proliferation of cells at the inner end of the proctodaeum. This extends 

 forward, in the form of a single layer of epithelium, along the dorsal 

 surface of the yolk past the stomodaeal invagination and also around the 

 sides of the yolk toward the ventral side of the egg. It appears that most 

 if not all of the mid-gut epithelium comes from the posterior end. If 

 the stomodaeum takes any part in the formation, it seems that it must be 

 only the fusing of the tip of the stomodaeal invagination with the sheet 

 of epithelium that has come from the proctodaeum. The epithelium 

 is regarded by Pratt as an ectodermal derivative. 



HYMENOPTERA 



Little is known of the embryology of the suborder Chalastogastra. 

 Graber (1890) says that in the barberry sawfly (Hylotoma herheridis) 

 entoderm rudiments are present at the blind ends of the stomodaeal and 

 proctodaeal invaginations, each of which develops the usual pair of rib- 

 bons that later form the mid-gut epithelium. 



Writers who have dealt with the development of the aculeate Hymen- 

 optera are in the main agreed that the mid-gut epithelium is derived 

 from bipolar mesenteron rudiments. Nelson (1915) found that in the 

 honeybee the two rudiments are transferred to the dorsal side of the 

 poles of the egg by the lengthening of the embryo. From the caudal 

 margin of the anterior rudiment and from the cephalic end of the pos- 

 terior rudiment a thin tongue-like dorsal process, or ribbon, is sent out, 

 these fusing when they meet at the anterior third of the egg. This 

 dorsal strip then widens, growing around the yolk, the lateral margins 

 uniting on the midventral line a short time before hatching. The floor 

 of the stomodaeum, although chiefly ectodermal, is formed also by cells 



