201) 



KiMIJRYOLOGY. 



kf 



mf 



continuous marginal furrow from the much more extensive extra- 

 embryonic area, which 

 serves for the formation 

 of evanescent organs like 

 the yolk-sac and the em- 

 bryonic membranes. 



hb , The marginal grooves 

 are formed by the infold- 

 ing of the outer germ-layer 



hi" and the parietal middle 

 layer, which are together 

 called the somatopleure, and 

 in such a manner that the 



A 



ridge of the original small 

 fold is directed downward 

 toward the yolk (Plate I., 

 fig. 8 sf). The space en- 

 closed by the two folded 

 layers is the marginal 

 groove (</r). As we have 

 distinguished on the latter 

 several regions, which are 

 developed at different times, 

 so must we here distinguish 

 the corresponding folds, 

 pr and we consequently speak 

 of a headfold, a tail- 

 fold, and the two lateral 

 folds. 



The keadfold ' appears, 

 first of all, even on the 

 first, but more distinctly 

 on the second, day of in- 

 cubation. By means of 

 it the head-end of the 

 embryonal fundament is 

 formed and separated from 

 the extra -embryonic part 

 of the germ-layers. At 

 the moment of its origin it is turned directly downward toward the 

 yolk; but the more it enlarges, whereby the anterior marginal 



Fig. 121. Blastoderm of the Chick, incubated 33 hours, 

 after DUVAL. 



< >ue sees the pellucid area, lif t surrounded by a portion 

 of the opaque area, <(/'. The fundament of the nervous 

 system is closed anteriorly and segmented into three 

 brain-resides, kb l , hb", hb'' ; belaud, the medullary 

 fold mf is stil] open. On either side of it lie six 

 primitive segments, us. The posterior end of the 

 fundament of the embryo is occupied by the primitive 

 streak with the primitive groove, pr. 



