YOUNG TWIN HUMAN EMBRYOS WITH 17-10 PAIRED SOMITES. 23 



the umbilical veins. A similar course has been noted for the allantois by Wallin (1913) 

 in an embryo of 13 somites. This position it retains throughout the rest of its course to 

 its termination, which is just before the chorion is reached. The allantois possesses a 

 lumen throughout, although it is almost obliterated in some parts of its course in the belly 

 stalk. 



On the ventral surface of the body in the median line, just behind the belly stalk, is 

 a shallow depression (plate 1 and plate 3, tig. 3) marking the position of the cloaca! mem- 

 brane, where the cloaca fuses with the ectoderm. The cloacal membrane is more than 2 

 cell layers in thickness, but the cells forming it lose their regular order, and are so irregular 

 in shape and position that no definite number of layers can be recognized. The cloaca fuses 

 with the ectoderm in two areas, both situated in the median line, with a free interval between, 

 in which mesoderm is found. Just behind the level of the second area of fusion the grooves 

 on the cloaca disappear and both bladder and intestinal bay open into a full rounded cavity 

 contained in the postanal gut. This part of the gut is very short and ends in a broad, blunt 

 surface, where the cells fuse with and are indistinguishable from the rest of the tissues in 

 the tail region, here forming the primitive streak. 



The alimentary system of Embryo V is different enough to merit a separate descrip- 

 tion and throws some very valuable light on appearances not very clear in its twin. The 

 stomodseum and buccopharyngeal membrane of Embryo V are similar to those of Embryo 

 VI, except that there is only one perforation of the membrane. As in the latter embryo, 

 no pouch of Rathke is present and no distinguishable pouch of Seessel. The first branchial 

 pouch is large and reaches to the ectoderm, with which it is only just coming into contact, 

 and there is thus only a very small area of fusion. The entrance into this pouch from the 

 pharynx is distinctly constricted. Near the distal end of the pouch 2 ridge-like villosities, 

 lying on opposite walls, project into the lumen and cut off a very small terminal chamber in 

 the pouch, communicating by the narrowed opening with the larger medial part of the 

 pouch. These villosities are similar to that mentioned in describing the first pouch* in 

 Embryo VI. There is also a single one present in the second pouch of Embryo V. These 

 villosities I interpret as being identical with the structure described by Grosser, in Keibel 

 and Mall's Embiyology, as being constantly present in the first gill pouch of all young 

 embryos of about this age, and which he interprets as a rudimentary internal gill. He 

 found this structure only in the first pouch, and evidently single, which is true here in the 

 case of Embryo VI, but in Embryo V there are 2 ridges on opposite walls in the first pouch 

 and 1 in the second pouch. If these represent internal gills, we would expect in favorable 

 cases to find two lying opposite in a pouch, and would also expect indications of them in 

 more than one pouch, so that the conditions found here ought to add considerable proof 

 to this suggestion as to their significance. 



The second branchial pouch is small and pointed and does not yet reach the ectoderm, 

 as in Embryo VI, but is still considerably separated from it. The third pouch is small, 

 pointed, and indistinct, and merged with the posterior part of the pharynx. In the median 

 line ventrally between the first and second pouches is seen the anlage of the thyreoid gland. 

 This is much more developed than in Embryo VI, and forms a large, broad pouch with a 

 slightly narrowed neck, but has still a large lumen. The epithelium is somewhat thick- 

 ened at the bottom of the pouch. This anlage lies directly caudad of the median ventral 

 aorta and projects down deeply behind it. 



Immediately in front of the thyreoid anlage the ventral aorta causes, as in Embryo 

 VI, an immense median bulging of the pharyngeal floor. A deep groove lies to each side 



