WITH SPECIAL REFERENCE TO THE VASCULAR SYSTEM. 71 



As regards the yolk-sac, even approximate dimensions can not be given, on 

 account of the extensive folding which has occurred here. It seems rather larger 

 in the models than in the photographs and is absolutely and relatively larger than 

 in No. 391, or in the Kromer specimen; in the latter the yolk-sac is quite small. 

 Figure 3 shows a short, well-marked diverticulum of the yolk-sac very near the 

 embryo. Its lumen is lined by high columnar cells, the whole knob-like structure 

 projecting into a mass of vascular mesenchyme. If it were shifted forward a frac- 

 tion of a millimeter into the septum transversum and nearer to the omphalomesen- 

 teric veins we would have, not one of the so-called glands or crypts of the yolk-sac, 

 but instead, the very beginning of hepatic development. As it is, one is reminded 

 of the hepatic function ascribed to the epithelial tubules and crypts of the yolk-sac 

 by Graf Spee (1896) and others. All things considered, we think that the umbilical 

 vesicle, so-called, may subserve functions other than those of a purely haemato- 

 poietic nature (cf. Eternod, 1909, 1913; Jordon, 1910). Very near the diverticulum 

 just mentioned is a smaller, solid outgrowth of the entoderm into the adjacent 



mesoderm. 



MESODERM AND CCELOM, SEGMENTATION. 



The mesoderm of the body of the embryo is in the form of two narrow, com- 

 pact bands, lying between the walls of the neural groove, the surface ectoderm and 

 the entoderm of the yolk-sac. Toward the neural folds it is best defined, its other 

 limits being much less sharp. Laterally it becomes looser and is continued into the 

 mesot helium covering the yolk-sac and amnion, while caudad it passes into the 

 diffuse mesenchyme of the primitive-streak region, where it is continued across the 

 median line. Cephalad these paired mesodermic masses break up to form the 

 scattered mesenchyme of the head and are also very clearly in connection with 

 the thickened pericardial mesothelium as well (text-fig. A). The mesoderm of 

 the body (the two compact bands as above noted) is in the first stages of segmenta- 

 tion. Its mesial border is rather uneven, but exhibits certain very definite, fairly 

 symmetrical, deep indentations which will later separate off, partially at least, the 

 somites of this region. 



Near the mesial margin of the mesodermic mass are formed the beginnings of 

 the so-called myoccels, very small or indicated only by the arrangement of the sur- 

 rounding cells. Of these minute closed cavities there are three on the left side and 

 two on the right, while between them are the indentations just mentioned. There 

 is no indication here of a lateral extension of the myoccels, either to help form, or 

 set up a secondary connection with, the intraembryonic ceelom, as is described by 

 Keibel and Elze (1906) in the case of the second somite in an embryo of 1.38 mm. 

 The embryonic ceelom is, except for the pericardial cavity and the small myoccels 

 just noted, practically non-existent as yet, but a transitory or inconstant connection 

 between it and the cavities of the somites may be established later. We have here, 

 therefore, evidence of three segments on one side and two on the other; of these 

 the middle or second segment on the left is the most distinct. The anterior limits 

 of the first segment on both sides and of the third on the left and the second on the 

 right are very uncertain (figs. 1 and 2). 



