188 



STUDY OF YOUNG CHICK EMBRYOS. 



with large vacuoles, left by yolk masses which the cells have digested; toward 

 the periphery vacuoles with partly absorbed yolk may be found. 



Section through the Venous End of the Heart (Fig. 137). The relations differ 

 but little from those in figure 136 except for the heart and the ganglionic crest. 

 The heart shows its bend toward the left side (the right in the figure), and in 

 this bend both the endothelial, Endo, and the mesothelial portions, m.Ht, partici- 

 pate. The mesocardium, x, is clearly recognizable, and comprises two mesodermic 

 lamina. By it the heart is suspended throughout its length from the ventral wall 

 of the fore-gut. The mesocardium soon disappears, and the mesothelial heart there- 

 upon becomes closed dorsally, and is attached only by its aortic and venous ends 

 to the neighboring tissues. A strand of cells from the endothelial heart passes 



Som. EC. 



Ao.D G. 



Md. 



Acv. 



m.Ht. 



x Endo. 



Ph. 



FIG. 137. SECTION OF A CHICK EMBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 130. 

 Acv, Ammo-cardiac vesicles. Ao.D, Dorsal aorta. EC, Epidermis. Endo, Endothelial heart. G, Ganglionic 

 crest. Md, Medullary tube (Hind-brain), mes, Mesenchyma. m.Ht, Mesothelial heart. Ph, Fore-gut. 

 Som, Somatopleure. Spl, Splanchnopleure. x, Mesocardium. X 100 diams. 



through the mesocardium and joins the entoderm. The significance of this junc- 

 tion is not clear. The ganglionic crest, G, is very distinct; it is overlaid by a thin 

 lamina of the epidermis, and is in texture quite unlike the brain-wall proper, Md. 

 The cells composing it are considerably individualized and somewhat separated from 

 one another by clear spaces. 



Figure 138 represents a section somewhat more highly magnified through the 

 heart anlage of a slightly younger embryo. The medullary groove, Md, is not 

 closed. The ccelom does not yet extend across the median line, but there is only 

 a thin partition separating the amnio-cardiac vesicles, Am.ves, from one another. 

 The mesothelial heart, msth, is a relatively thick layer, thrown into irregular folds. 

 The endothelial heart is represented only by a few scattered angioblastic cells, 

 Endo, which as yet show no definite order. 



The further development of the heart may be understood by the examination 

 of a somewhat older stage (Fig. 139). As shown in the illustration, the mesothe- 

 lium has become very protuberant, m.ht, in the median line underneath the fore- 

 gut, Ph. On either side it rapidly thins out, ,msth. In the protuberant fold we 



