COMPLETION OF THE FORM OF THE BODY 



555 



Completion of the definite form of the body. This is accomplished 

 by the growth of the primitive band around the yolk, the band 

 widening, so that its edges behind the head extend up, and finally 

 meet on the back, forming the back or tergum of the embryo, thus 

 enclosing the yolk (Fig. 530, F). The tergal wall of the head is 

 due to the dorsal growth of the head-lobes, and of the clypeo-labral 

 region. In the course of this process the anterior end of the primi- 



FIG. 530. Diagram of the formation of the dorsal organ in Hydrophilus. A, cross-section 

 through an egg, whose primitive streak is still covered over by amnion (a) and serosa (). , amnion 

 and serosa are grown together in the middle line, then separated and drawn back to form a fold on 

 each side. C, by the contraction of the serosa (x), which becomes converted into the dorsal plate, 

 the folds become drawn up dorsally. Z>, the contracted serosa becomes partly overgrown by the 

 folds. E, the folds grow together to form the dorsal tube. F, the mid-gut has "closed over dorsally 

 and enclosed the dorsal tube () : a, amnion ; d, yolk ; ec, ectoderm ; ft, heart ; I, body-cavity ; 

 m, rudiment of the mid-gut ; n, nervous system ; x, serosa (in C and D = dorsal plate, in #and f, 

 dorsal tube) ; tr, the chief tracheal stem. After tiraber and Kowalevsky, from Lang, and Kor- 

 schelt and Heider. 



tive band becomes turned up dorsally, forming a dorsal curve or 

 bend. By this bending up of the primitive band the forehead near- 

 est the mouth forms a transverse ridge, the labrum, while the basal 

 or earlier part of the forehead now is differentiated into the clypeus. 

 This clypeo-labral region likewise forms the roof or palatal region 

 of the mouth. The head-lobes cause by this dorsal growth a rotat- 

 ing motion which carries the rudimental antennae back over the 

 mouth. 



