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



of this, while a shallow one lies in the median line on the crest of the elevation. This third 

 groove disappears, when followed caudally, on the down slope of the elevation, but the 

 lateral grooves unite behind the elevation and then become, suddenly the deep median 

 groove or trough, forming the thyreoid, with its opening into the pharynx slightly con- 

 stricted by the lips of the groove. These 2 lateral grooves have never been described in 

 any other embryo, but their presence here is not an abnormal phenomenon. It is due to 

 the immense size of the aortic stem that the floor of the pharynx is heaved up, forming a 

 temporary stage of no special consequence, and this elevation and its consequent lateral 

 grooves will quickly disappear, so that in stages such as shown by Van den Broek's and 

 Thompson's embryos, the pharynx does not exhibit these peculiarities, and these grooves, 

 being only incidental to the formation of the median elevation, are no longer found con- 

 verging to form the beginning of the thyreoid gland. 



The gut back of the gill pouches, as far as the yolk sac, is much compressed dorso- 

 ventrally, just as in Embryo VI, and consequently no esophageal and gastric region, which 

 is in these early stages laterally compressed, can be recognized. In this portion of the gut 

 the dorsal surface is smooth, the sides pointed, and the ventral surface wavy in outline on 

 section. This latter condition is due to 3 or 4 longitudinal grooves on the wall. As the 

 yolk sac is approached and the ventral wall begins to slope down behind the sinus venosus, 

 a distinct median bay or outpouching forward is seen, which is the anlage of the liver (the 

 liver bay). The communication between yolk sac and intestine is somewhat constricted 

 to form a yolk stalk, so that this embryo does not lie wide open over the sac, as Embryo VI 

 does. The hindgut is oval in cross-section, and passes insensibly into the cloaca, which is 

 not quite so well developed as in Embryo VI, the main difference being that the cavity is 

 not yet divided into two, although indications are present of an approaching differentiation. 

 The stages here are valuable in showing the condition just previous to division. Here the 

 lumen is large and capacious and on section appears triangular, showing a broad dorsal 

 area and a ventral pointed portion (text fig. 7 c). On the lateral wall, in the broad portion, 

 is a shallow furrow on each side, forming a small ridge projecting into the lumen. It is by 

 the deepening of this furrow that the condition in Embryo VI is obtained, with the narrow 

 dorsal rectal bay and the lozenge-shaped ventral bay almost completely separated. Fusion 

 along the line of these opposing ridges would completely separate the rectum from the 

 ventral part of the cloaca. The furrows outside, and consequent ridges inside the cloaca, 

 are not well developed in Embryo V, but start about the level of the origin of the allantois 

 and extend back beyond the level of the cloacal membrane, where they disappear. The 

 postanal gut is very short and it very soon fuses with the tissues of the primitive streak. 

 The cloacal membrane is marked by a depression on the outside of the body and the endo- 

 derm of the gut comes into contact and fuses with it for a short distance, and there are 

 thickening and change in the epithelial layers of both ectoderm and endoderm. 



The allantois, arising from the cloaca, runs cephalad, then ventrally into the belly 

 stalk, where it lies for a long time ventral to the umbilical arteries, but finally, as in Embryo 

 VI, passes through a loop formed by the arteries to lie dorsal to them. It is formed of a 

 single layer of endoderm. The lumen is almost obliterated just after entering the stalk, and 

 then becomes very large as the allantois lies under the umbilical artery, but becomes small 

 again as it goes through the loop. The termination of the allantois is just at the chorion. 



The remains of the neurenteric canal, as described under the notochord, fuse with the 

 cloaca dorsally. 



