22 YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 



of 13 somites, but Low describes 4 in one of 14 somites, while here only 3 are present. 

 Again, Van den Broek and Thompson, in embryos of 22 and 23 somites, find only 4, the 

 fourth being small, so that evidently the fourth in Low's embryo is precocious in its appear- 

 ance. 



The median thyreoid anlage (plate 1 and plate 3, fig. 2) is present in this embryo and 

 is very similar to that described by Low, being a rather wide shallow depression in the 

 median line ventrally between the first and second gill pouches. In front of this is the 

 large elevation of the floor of the pharynx caused by the underlying ventral aorta as it turns 

 forward and divides into the paired aortae. Wallin also found the median thyreoid anlage 

 just caudad of the ventral aorta. 



The relation of the notochord to the pharynx has been already fully described. 



In the young embryos both older and younger than Embryo VI, the alimentary canal 

 caudal to the pharynx becomes laterally compressed, forming a narrow dorsoventral pas- 

 sage giving rise to esophagus and stomach. This is not so in this embryo, where the 

 gut slightly narrows laterally from the third gill pouch backward, still retaining a consider- 

 able lateral diameter, but showing the remarkable condition of being very markedly 

 compressed dorsoventrally (text fig. 7 a), so that the lumen is almost slit-like. Over the 

 sinus venosus this condition changes, as the gut begins to widen dorsoventrally again, and 

 then it opens rapidly into communication with the yolk sac. On the ventral wall as it 

 slopes toward the yolk sac, just behind the sinus venosus, a shallow depression runs forward, 

 forming the liver bay, which is in about the same stage as seen in Low's embryo (1908). 



No tracheal groove or lung anlage can be distinguished. 



The opening of the yolk sac (plate 1) comprises one-third the total length of the 

 embryo, and here the embryo is flattened out and there are no lateral constrictions and no 

 way of locating the dividing line between yolk sac and body of the embryo. Back of this 

 passes the hindgut, at first rounded in cross-section, but rapidly becoming ovoid or pear- 

 shaped (plate 3, fig. 3), with the small end dorsally. As the cloaca (plate 3, fig. 3) is reached, 

 a groove (text fig. 7 b) appears on each side between the large and small ends of the ovoid 

 and almost cuts the cloaca into two separate chambers throughout its whole length. There 

 is, however, a very narrow chink throughout the extent of the cloaca, where the two parts 

 of the lumen (the rectal bay and the bladder bay) are in continuity. The rectal part of 

 the cloaca is thus much compressed laterally and has a small oval lumen. The bladder 

 portion is lozenge- or diamond-shaped, with dorsal, ventral, and two lateral angles, and the 

 lumen of this part is very large. From this embryo it appears that the separation of the 

 ventral part of the cloaca from the rectum is accomplished by the cutting in of two lateral 

 grooves between them, and not by the extension caudally, as claimed by Felix (1912), of 

 a gradually deepening saddle fissure starting just cephalad of the origin of the allantois. 

 It also occurs much earlier than stated by Felix, who says the separation begins in embryos 

 of 4.9 mm. nape length or 5.3 mm. greatest length. There is actual contact of the sides 

 and every possibility of fusion along the lines of lateral constriction. The saddle-like 

 constriction described by Felix, starting above the allantois, is not yet here indicated on 

 the ventral wall of the gut, but the allantois quite clearly comes off the most cephalad 

 portion of the ventral part of the cloaca. It runs ventrally (plate 1 and plate 3, fig. 3) in 

 a gentle curve out into the belly stalk, with an umbilical artery on either side of it. The 

 arteries unite in a blood reservoir above it in the stalk, where it at first lies ventral to all 

 other structures. The common artery soon splits, however, and then reunites, forming 

 a loop through which the allantois passes to a position dorsal to the artery and ventral to 



