142 A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



thick ventral wall taking place gradually on the sides of the tube. The pulmonary 

 diverticulum is two to three cell-layers thick, the hepatic diverticulum three 

 to four. 



In the region of the yolk-stalk the epithelium is made up of one layer of 

 cubical or somewhat flattened cells. In the yolk-sac the epithelium is not every- 

 where the same. In some places the cells are large and cubical; in other places 

 flattened and less distinct. Often is it impossible to determine whether or not 

 an epithelium is present, for in such places the epithelial cells can not be distin- 

 guished from those of the mesenchyma. The mesenchyma surrounding the 

 yolk-sac also varies in thickness. It contains numerous blood-vessels and is 

 covered by the mesothelium of the body-cavity. 



The hind-gut, down as far as the cloaca, is composed of but a single layer of 

 cuboidal cells, which is of equal thickness all around. In the cloaca and caudal 

 intestine the epithelium of the side and ventral walls is thickened, and is com- 

 posed of two to three layers of cells. The entodermal epithelium of the cloacal 

 membrane shows no distinguishing characteristics. It abuts against the ecto- 

 dermal epithelium, but both layers can be made out distinctly. 



SOMITES. 



In all, 24 pairs of somites are present; they extend from the lower end of the 

 hindbrain to a little beyond the point where the allantoic duct passes out from 

 the cloaca. The first somite is found in the region of the eighth and ninth rhom- 

 bic neuromeres. According to studies on a slightly older human embryo and on 

 young embryos of the pig, sheep, and cat, I have found that this position is 

 normally occupied by the second somite. Watt 48 likewise shows the second 

 somite in this position, the first somite being in relation to the seventh and eighth 

 neuromeres. I have looked repeatedly in my embryo, however, for evidence of 

 another somite in front of the one which I have designated as the first, but have 

 been unable to find any definite indication of such. 



As in other young embryos which have been described, the different pairs of 

 somites are found in different stages of development, those nearer the head end 

 always being more advanced than those behind them. In the caudal end of the 

 embryo are found somites in the process of formation; in the head they are already 

 partially broken up. Following Ingalls's 19 plan, I shall begin my description with 

 the somites of the tail and proceed forwards. 



The caudal end of the vertebral plates of mesoderm fill up entirely the tail of 

 the embryo around the neural tube, notochord, and tail gut. In the region of the 

 cloacal membrane they appear as solid masses or cords of mesoderm with closely 

 packed cells. At their cephalic ends a small cavity is apparent. Another pair of 

 somites, the twenty-fifth, are partially formed by an incomplete transverse furrow. 

 Numerous mitotic figures are present in the vertebral plates. 



The twenty-fourth somite (second lumbar) is in a very early stage of develop- 

 ment. It is somewhat cubical in shape and in its center is a distinct cavity, the 

 mvoecele. The walls of the somite, which may be described as dorsal, ventral, 



