THE PRIMITIVE BAND 



539 



there are two opposite types, though connected by transitional forms. In 

 the one case the primitive band, i.e. its ventral portion, the "ventral plate" 



FIG. 517. Primitive streak of a lepidppter in cross-section : ah, amniotic cavity ; am, amnion ; 

 c, coelomic cavity; do, nutritive yolk, divided into single nucleated masses; ec, ectoderm; m, 

 mesoderm; pr, primitive thickenings of the ventral nervous cord; s, serosa. Combined figure 

 after those of Brobretsky and llatschek, from Korschelt and Heider. 



(Fig. 518, 6, p) is pushed in or invaginated in the interior of the egg ; in the 

 other case it becomes overgrown by the folds of the amnion arising from its 

 edges. 



A B C a D E 



FIG. 518. Five diagrammatic mediansections representing the growth of a dragon-fly (Calop- 

 teryx). A-C, development of the primitive streak (k, k') by invagination. D, the amnion-fold (a/), 

 growing over the head-end of the primitive streak. E, closing of the opening of the amnion-cavity 

 (ah): v, ventral, d, dorsal side ; a, fore, 6, hind end of egg ; t>l, blastoderm ; bp, ventral plate ; do, 

 yolk ; k, head-end, k' ', caudal end, of the primitive streak ; kh, germinal thickening or Initial point 

 of invagination ; , serosa. After Brandt, from Korschelt and Heider. 



In insects with an overgrown primitive band, the band at the beginning is 

 generally short and always situated on the ventral side of the egg, with the 

 head-end looking forward, and remains in this position throughout embryonic 



