TRACHEATA. 



405 



fig. 185 B, do. The canal is buried on the dorsal part of the yolk, but for 

 some time remains open by a round aperture in front (fig. 184 C). The 

 whole structure is known as the dorsal canal. It appears to atrophy without 

 leaving a trace. The heart when formed lies immediately dorsal 



A. 

 B. 

 C. 



P'iG. 185. THREE TRANSVERSE SECTIONS THROUGH ADVANCED 



EMBRYOS OF HYDROPHILUS. 



Section through the posterior part of the body of the same age as fig. 184 A. 

 Section through the embryo of the same age as fig. 1 84 C. 

 Section through a still older embryo. 



do. dorsal plate ; v n. ventral nerve cord ; al. mesenteron ; ht. heart. 

 The large spaces at the sides are parts of the body cavity. 



In the Poduridae the embryonic membranes appear to be at any rate 

 imperfect. Metschnikoff states in his paper on Geophilus that in some ants 

 no true embryonic membranes are found, but merely scattered cells which 

 take their place. In the Ichneumonidas the existence of two embryonic 

 membranes is very doubtful. 



Formation of the embryonic layers. The formation of the 

 layers has been studied in sections by Kowalevsky (No. 416), 



1 According to Kowalevsky the history of the dorsal plate is somewhat different. 

 He believes that on the absorption of the amnion the ventral plate unites with the 

 serous membrane, and that the latter directly gives rise to the dorsal integument, 

 while the thickened part of it becomes involuted to form the dorsal tube already 

 described. 



