EMBRYONIC DEVELOPMENT. 



545 



marked off from the rest, and forms the structure known as the 

 ventrtif f,lntp., which constitutes the first rudiment of the head and 

 ventral half of the embryo. 



In many cases {Hlnjin-ltota Lil><-Util) the ventral plate grows out 

 from a hill-like thickening of the blastoderm (fig. 454) into the 

 interior of the yolk, so that an internal ventral plate arises, in the 

 formation of which a portion, though a small one, of the external 

 blastoderm participates. The ventral thickening, which gives rise to 

 the ventral plate, is caused by long columnar cells, and is at first 

 confined to a small portion of the egg; in Hydrophilus the posterior 

 end (fig. 455, a). Inasmuch as its lateral edges become elevated 



FIG. 45-k Embryonic development of Ctilo/ifcri/x vlrgo (after Al. Brandt). , Commencing 

 involution of the ventral plate. The blastoderm was at first one-layered and thickened at 

 the poles. G, edge of ventral plate, it, Later stage of the involution, c, The embryonic 

 membranes are developed ; L/>, parietal (serosa); La, visceral (amnion) layer of the latter. 

 (7, The appendages have sprouted out on tlie ventral plate. A, Antenna ; Mil, mandible ; 

 M.i-', first maxilla ; M.I-, second maxilla (labium or lower lip). Then follow three pairs of 

 legs, c, Eversioii of the embryo which is protruded from, the sheath of the visceral layer. 

 (', Completion of the inversion ; the hind end of the body is free ; the yolk sac is on the 

 ilorsal surface. 



and grow towards one another (fig. 455, b,c), the thickened ventral 

 plate first assumes the form of a groove, and then, after the fusion of 

 the lateral edges, becomes a, canal, the lumen of which is soon 

 obliterated. The roof only of this canal corresponds to the epiblast. 

 while the cells of its floor and its sides give rise to the first rudiment 

 of the mesoblast. At the edge of the so-called ventral plate, fresh 



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