336 INSECT A. 



the wall of which in the process appears to be folded over so as first of 

 all to form a groove and finally a canal. The stages in this growth are 

 shewn from the surface in fig. 184 B and C and in section in fig. 185 B, do. 

 The canal is buried on the dorsal part of the yolk, but for some time re- 

 mains 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 to it 1 . 



In the Poduridaj 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 Ichneumonidse 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), Hatschek 

 (No. 414), and Graber (No. 412), etc. From their researches it would 

 appear that the formation of the mesoblast always takes place in a 

 manner closely resembling that in Hydrophilus. The essential 

 features of the process (figs. 177 and 178) appear to be that a groove 

 is formed along the median line of the ventral plate, and that the 

 sides of this groove either (1) simply close over like the walls of 

 the medullary groove in Vertebrates, and so convert the groove into 

 a tube, which soon becomes solid and forms a mass or plate of 

 cells internal to the epiblast ; or (2) that the cells on each side of the 

 groove grow over it and meet in the middle line, forming a layer 

 external to the cells which lined the groove. The former of these 

 processes is the most usual ; and in the Muscids the dimensions of 

 the groove are very considerable (Graber, No. 411). In both cases 

 the process is fundamentally the same, and causes the ventral plate 

 to become divided into two layers 2 . The external layer or epiblast is an 

 uniform sheet forming the main part of the ventral plate (fig. 178 

 B, ep). It is continuous at its edge with the amnion. The inner 

 layer or mesoblast constitutes an independent plate of cells internal 

 to the epiblast (fig. 178 B, me). The mesoblast soon becomes divided 

 into two lateral bands. 



The origin of the hypoblast is still in dispute. It will be remem- 

 bered (vide pp. 94 and 95) that after the segmentation a number of 

 nuclei remain in the yolk ; and that eventually a secondary segmenta- 

 tion of the yolk takes place around these nuclei, and gives rise to a 

 mass of yolk cells, which fill up the interior of the embryo. These 

 cells are diagrammatically shewn in figs. 181 and 189, and it is 

 probable that they constitute the true hypoblast. Their further 

 history is given below. 



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. 



8 Tichomiroff (No. 420) denies the existence of a true invagination to form the 

 mesoblast, and also asserts that a separation of mesoblast cells from the epiblast can 

 take place at other parts besides the median ventral line. 



