190 JOHN LEWIS BREMER 



namely pig, rabbit, sheep, cat, and man, the two former have 

 relatively a large Wolffian body, both in length and thickness, 

 from a very early embryonic period. The increasing bulk of 

 Wolffian body itself is enhanced by the consequent increase in 

 the subcardinal veins, and these two factors cause a great pro- 

 trusion of the organ into the coeloni. The glomeruli, occupying 

 the ventro-mesial border of the organ, are thus moved from their 

 original position lateral to the aorta to one far ventral; and the 

 mesonephric arteries, running to the glomeruli, are lengthened 

 and arise from the ventro-lateral surface of the aorta, instead of 

 from the lateral. This is true, to some extent, in all mammals, 

 but occurs earlier and is exaggerated in the pig and the rabbit. 

 The horizontal anastomoses of the plexus leading dorsally from 

 the mesonephric arteries either do not form, or if already present 

 are strained until they become obliterated, leaving the meso- 

 nephric arteries unconnected with the plexus, and far ventral 

 to the area in which the kidney is developing (see diagram, 

 fig. 7). 



The fact that the spermatic artery is originally a branch from 

 one of the arteries to the glomeruli of the Wolffian body, or as 

 stated for the pig b}^ Hill (6), a new vessel of the same type and 

 with a similar course, gives us a check on the origin of the renal 

 artery, for, if the renal artery is connected in any way with the 

 mesonephric plexus, it will be common to find, as an anomaly, 

 the spermatic artery as its branch. Such a connection I have 

 not found described for either rabbit or pig. 



A second factor which distinguishes different types of embryos 

 is the greater or lesser curvature of the rump region. A compari- 

 son of the profile drawings of cat and rabbit embryos of about 

 10.0 mm. or younger with those of man, sheep, or pig of the 

 same age shows in the former a strikingly increased curvature 

 of the back just anterior to the pelvic limb buds (fig. 6). A 

 study of sections of these embryos shows that the aorta does not 

 follow this curve closely, but takes a shorter path, and so lies 

 here further from the spinal cord than in the thoracic region. 

 This causes the dorsal segmental arteries of the lumbar aorta, 

 which run to the spinal cord, to assume a course almost di- 



