EMBRYO WITH EIGHT SEGMENTS. 185 



its own disappearance creates the oral opening of the fore-gut. Close to the fore-gut 

 lie four blood-vessels, two above and two below, the dorsal, Ao.d, and ventral, 

 Ao.v, aortae, respectively. The dorsal vessels are much the larger. If the series 

 of sections be followed through cephalad the ventral aorta will be found, before 

 the tip of the fore-gut is reached, to bend dorsalward and join the dorsal aorta of 

 the same side. If the series Tbe followed through in the caudad direction, it will 

 be observed that the two ventral aortae draw toward the median line until they 

 meet and unite in a single trunk, the main aorta, which is continuous with the 

 heart (Fig. 135, Hf). It is thus learned that the blood leaves the heart at its cepha- 

 lic end by a single channel, which soon divides; the branches curve upward and 

 pass to the dorsal side of the pharynx, forming two dorsal channels conducting 

 the blood-stream backward. The blood-vessels consist each of a very thin layer 

 of cells, epithelial in character and termed endothelium; the nuclei are flattened 

 and therefore appear oval in section. All the remainder of the section is occupied 

 by loosely scattered cells, which are of two sorts: first, those marked mes, which 

 till the ventral and lateral regions, and constitute the true mesenchyma; the mesen- 

 chymal cells have nuclei with small amounts of protoplasm around them, and 

 strands of protoplasm connect the cells together; there are no cell boundaries; 

 the- t'ssue might be described as an irregular reticulum with nucleated nodes; 

 second, those cells marked G, which form two lateral groups on the dorsal side 

 adjoining the mid-brain; these groups have been named the ganglionic crests by 

 some writers, mesectoderm by others. The cells in question resemble those of the 

 true mesenchyma, but have more protoplasm around the nuclei and appear 

 therefore more deeply stained than the mesenchyma proper. If the cells of the 

 crest be followed dorsally they will be seen to form a narrow band which joins the 

 ectoderm near the median line, and by following the sections headward, the crests 

 will be fouHd to merge with ectodermal thickenings (Fig. 133, G). From these 

 relations it has been inferred that the crest on each side arises from a local pro- 

 liferation of the ectoderm. The crest is easily seen in surface views of stained 

 chicks (Fig. 131, G). Two principal views as to the future of the crest cells of the 

 mid-brain region have been brought forward: first, that they are true ganglionic 

 anlages, which disappear by autolysis; second, that they are converted into true 

 mesenchyma. 



Section through the Hind-brain (Fig. 135). The head is no longer free, but 

 fuses laterally with the layers of the germinal area; hence the ectoderm, EC, instead 

 of bending over on to the ventral side, bends in the opposite direction away from the 

 embryo. The mesoderm stretches across the median line under the embryo. There 

 is a large space, Coe, in the mesoderm; the space is part of the primitive body-cavity 

 or ccelom; it extends completely across the embryo and out into the germinal area 

 on each side. The ccelom is everywhere bounded by a thin epithelial layer, msth, 

 the mesothelium, which at this stage resembles an endothelium as seen in section; 

 it forms one part of the mesoderm, the bulky mesenchyma forming the other part. 



