YOUNG TWIN HUMAN EMBRYOS WITH 17-10 PAIRED SOMITES. 



21 



tion of the openings except a slight one for the first. The first pouch is very large and runs 

 laterally and dorsally out to come into contact with the ectoderm, with which it is fused 

 in two areas for a very short distance. The cephalocaudal diameter of the pouch is much 

 greater than the dorsoventral. and its cavity is wide and capacious, hut as yet undivided 

 into dorsal and ventral portions. There is a very peculiar condition found here, however, 

 for just before the lateral edge of the pouch is reached, a narrow villus-like ridge, filled with 

 mesoderm, rises from the ventral wall of the pouch into the cavity, cutting off a narrow 

 gutter in the extreme lateral part of the pouch. From the appearance of the sections this 

 ridge is evidently not due to an infolding of the wall as the result of sectioning or mounting. 



S> Diagram of nephrogenic system of Embryo VI. 



t>. Diagram of nephrogenic system of Embryo V. Tho numbers in text d^ures 5 and indicate the level of the somites. 

 7 A, lamera-lucida outline of the alimentary canal in Embryo VI. just cephalad of th. ' . • 130. 



7b. lamera-lucida outline of the cloaca in Embryo VI. at the level of the cloacal membrane. Magnification X 120. 

 7c. (.'amera-lucida outline of the cloaca in Embryo V. at the level of the cloacal membrane. Magnification \ 120. 

 For list of abbreviations see page 40. 



The second pouch is not nearly so large as the first, but also reaches the ectoderm, with 

 which it is fused over two small area-, with a narrow interval between. This pouch is 

 wide for some distance as it leaves the pharynx, but only the most dorsal part reaches the 

 ectoderm, appearing as a tapering, conical process of the larger portion. The third pouch is 

 differentiated in front from the second by the presence oi the intervening branchial arch, 

 but can not be delimited behind, where it merges into the pharynx. This pouch does not 

 extend far laterally, and so does not reach the ectoderm. It is somewhat pointed, as was 

 the case with the second. Wallin (1913] describes only "J pairs oi gill pouches in an embryo 



