478 Description of a 4-mm. Human Embryo 
aorta, but separated from it by the mesoderm of the mesentery, which 
has been dissected away to the median line. Ventral to the entodermic 
intestine, also, the mesoderm of the intestinal wall has been cut away to 
the median line. The entodermic tube follows the general curve of the 
embryo, growing gradually larger, and empties into a large rounded 
pouch, also compressed laterally, the cloaca. In Fig. 8 the section passes 
nearly through the junction of the intestine and the cloaca, and the com- 
pressed shape can be more easily seen. The mesentery becomes gradually 
shorter and finally disappears entirely. The cloaca, along its greater 
curvature, lies close to the notochord, and passes into the tail as the tail 
gut, which soon loses its lumen and ends near the tip of the tail by fusing 
with the ectoderm in an undifferentiated mass. At the root of the tail 
the cloaca comes into close contact with the ectoderm of the skin at the 
anal plate (An. pl.). From the upper, posterior corner of the cloaca, 
just above the entrance of the intestine, passes off the allantois, which, as 
a slender rounded tube, curves out into the body stalk close to the wall of 
the ccelom, between and posterior to the two umbilical arteries, and ends 
blindly. A slight, irregular swelling occurs in its course, marking per- 
haps the position of the future bladder. 
Wolffian duct—The Wolffian duct on the right side (Figs. 15, 16, 
W. d.) is shown in the model standing out at the side, because freed from 
the mesoderm. It follows the general curve of the body, just ventral 
to the plane of the notochord, gradually approaches the median line on its 
way toward the tail, and ends before reaching the level of the anal 
plate by joining the cloaca (W. d.’). As can be seen in Fig. 8, its position 
corresponds with the external groove between the area of primitive seg- 
ments and the body wall. On the left side the Wolffian duct does not ex- 
tend so far as the cloaca, but ends blindly, and so is cut only once in 
the section (Fig. 8, W. d.). Dorsally the duct is oval in cross section, 
with long axis parallel with the lateral surface of the Wolffian ridge, 
which is seen in Fig 8, projecting into the ccelom dorsally; but nearer 
its union with the cloaca, the duct assumes a rounded form. 
The Wolfian tubules, seen in Fig. 8, just mesial to the duct, are 
not shown in the model. They are short, sometimes with an S-shaped 
curve, frequently overlapping, as in the section given, where portions of 
two tubules are shown on the left side (W. ¢.). In all there are twenty- 
eight tubules, arranged in two groups; one caudal group, consisting of 
twenty-three tubules, close together, but not definitely arranged in rela- 
tion to the primitive segments or muscle plates (Fig. 8, musc.). This 
represents the true Wolffian body, and the tubules are all closed at their 
