TRACHEATA. 



397 



176 A, gg). The cells 

 which form the floor 

 of the groove are far 

 more columnar than 

 those of other parts 

 of the blastoderm (fig. 



177 A). The two 

 folds on each side of 

 it gradually approach 

 each other. They do 

 so at first behind, and 

 then in the middle; 

 from the latter point 

 the approximation 

 gradually extends 

 backwards and for- 

 wards (fig. 176 B and 

 C). In the middle 

 and hinder parts of 

 the ventral plate the 

 groove becomes, by 



FlG. 177. TWO TRANSVERSE SECTIONS THROUGH 



EMBRYOS OF HvDROPHiLUs piCEUS. (After Kowa- 



levsky.) 



A. Section through an embryo of the stage repre- 

 sented in fig. 176 B, at the point where the two 

 germinal folds most approximate. 



B. Section through an embryo somewhat later 

 than the stage fig. 1 76 D, through the anterior region 

 where the amnion has not completely closed over the 

 embryo. 



gg. germinal groove ; 

 nion ; yk. yolk. 



inc. mesohlast ; am. am- 



the coalescence of the folds, converted into a canal (fig. 178 A, 

 gg), the central cavity of which soon disappears, while at the 

 same time the cells of the wall undergo division, become more 

 rounded, and form a definite layer (me) the mesoblast beneath 

 the columnar cells of the surface. Anteriorly the process is 

 slightly different, though it leads to the similar formation of 

 mesoblast (fig. 177 B). The flat floor of the groove becomes in 

 front bodily converted into the mesoblast, but the groove itself 

 is never converted into a canal. The two folds simply meet 

 above, and form a continuous superficial layer. 



During the later stages of the process last described remark- 

 able structures, eminently characteristic of the Insecta, have 

 made their first appearance. These structures are certain 

 embryonic membranes or coverings, which present in their mode 

 of formation and arrangement a startling similarity to the true 

 and false amnion of the Vertebrata. They appear as a double 

 fold of the blastoderm round the edge of the germinal area, 

 which spreads over the ventral plate, from behind forwards, in a 



