EMBRYO WITH EIGHT SEGMENTS. 



191 



half of the medullary tube and fuses in the mid-dorsal line with the ganglionic 

 crest, G, which in its turn fuses with the medullary walh The crest, though not 

 large or conspicuous, can be distinguished readily. The mesenchyma, is clearly 

 differentiated only above the fore-gut, and on either side just beyond the lateral 

 boundary of the fore-gut it fuses with the mesotheliurn. > 



Som. 



Ao.D. Md. 



G. 



mes. EC. 



Spl. Ent. nch. ' a. V.om. 



FIG. 141. SECTION OF A CHICK EMBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION -144. 

 a, Groove corresponding to the prolongation of the lateral portion of the fore-gut. Ao.D, Dorsal aorta. EC, 

 Epidermis. Ent, Entoderm. G, Ganglionic crest. Md, Wall of Hind-brain, mes, Mesenchyma. 

 nch, Notochord. Som, Somatopleure. Spl, Splanchnopleure. V.om, Omphalo-mesaraic veins. X 100 

 diams. 



Section, behind the Fovea cardiaca and in- Front of the First Segment (Fig. 141). 

 The closed entodermal cavity of the embryo has become open and communicates 

 freely with tHe yolk-cavity. The omphalo-mesaraic veins occupy a more lateral 

 position. Otherwise the section differs so little from figure 140, that it does not call 

 for special description. * 



Som. 



EC. 



Md. 



Spl. Cce. Ao.D. nch. Ent. Msth. 



. FIG. 142. SECTION OF A CHICK EMBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 162. 

 Ao.D, Dorsal aorta. Ch, Ccelom. EC, Ectoderm. Ent, Entoderm. G, Anlage of Ganglionic crest, mes, 



Mesenchyma of the intersegmental cleft. Msth, Mesodermic lining of the coelom. nch, Notochord. Som, 



Somatopleure. Spl, Splanchnopleure. X 100 diams. 



Section between the First and Second Segments (Fig. 142). We are still in the 

 region of the hind-brain, which extends to the fourth or last occipital segment. 

 There is no distinction or limit between the embryonic and the vitelline divisions 

 of the archenteron. The hind-brain, Md, is oval in section; on its dorsal side it 

 fuses with the ganglionic crest, G, which seems now rather a part of the brain-wall 



