200 



EMBRYOLOGY. 



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. 



The marginal grooves 

 are formed by the infold- 

 ing of the outer germ-layer 

 and the parietal middle 

 layer, which are together 

 called the somatopleure, and 

 in such a manner that the 

 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 (gr). As we have 

 distinguished on the latter 

 several regions, which are 

 developed at different times, 

 so must we here distinguish 

 the corresponding folds, 

 and we consequently speak 

 of a headfoldj a tail- 

 fold, and the two lateral 

 folds. 



The headjold 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. 



One sees the pellucid area, hf, surrounded by a portion 

 of the opaque area, df. The fundament of the nervous 

 system is closed anteriorly and segmented into three 

 brain- vesicles, hb l , hb', hb a ; behind, the medullary 

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

 primitive segments, ut. The posterior end of the 

 fundament of the embryo is occupied by the primitivi- 

 streak with the primitive groove, pr. 



