YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 35 



again at the reflection on to the lateral body wall, so that evidently its thickening at the 

 root of the mesentery is not significant, although it is heavier than the other thickened por- 

 tions and occasionally is irregularly heaped up in more than 2 layers at points just medial 

 to the pronephros. 



THE SEPTUM TRANSVERSUM AND THE CCELOM. 



The septum transversum is very much like the one described by Peter Thompson 

 (1908) for the 23-somite stage, but is situated (plate 1) further cephalad, being opposite 

 the first 2 somites in this case. It is also not inclined obliquely from the dorsal surface 

 caudally and ventrally, but is nearly horizontal. It is convex ventrally and immediately 

 behind it is the yolk sac. In the dorsal portion the liver bay lies embedded. The septum 

 is completely attached to the body wall laterally and ventrally, but not dorsally, for to 

 each side of the gut lies the pleuroperitoneal passage of the ccelom, passing dorsal to the 

 septum. To the dorsal body wall the septum is attached by two lateral horns, one lateral 

 to each pleuroperitoneal passage, and in the median line it fuses with the mesoblast surround- 

 ing the gut. Its cephalic surface dorsal to the sinus venosus has attached to it the dorsal 

 mesocardium, which latter is prolonged forward from it in the median line, over the small 

 free portion of the sinus venosus. 



Contained in the tissues of the septum transversum are posteriorly the vitelline and 

 umbilical veins of each side, which enter it from the lateral body walls, while passing laterally 

 over the pleuroperitoneal passage and running ventrally into the septum, lateral to the 

 passage, is the duct of Cuvier of each side. The vitelline and umbilical veins of each side 

 unite into an immense common trunk in the septum, and with this trunk unites the duct of 

 Cuvier, the total union forming in the septum the horns of the sinus venosus. These 

 horns are united by the median, transverse, crescentic part of the sinus, which is embedded 

 in the anterior part of the septum, except a small portion which escapes from the septum 

 just before opening into the atrium. 



The pericardial portion of the ccelom has been described in connection with the heart. 

 It is large and capacious and forms one large chamber, except dorsally over the atrium and 

 again over part of the bulbus cordis and over the aortic stem, where the dorsal mesocardium 

 subdivides it into two. The most cephalic part of the cavity is that surrounding the bulbus 

 cordis, where the cavity reaches a level just in front of the beginning of the otocyst. This 

 corresponds exactly with the findings of Mrs. Gage (1905). The pericardial cavity rapidly 

 narrows posteriorly to each side of the dorsal mesocardium, and from this portion the pleuro- 

 peritoneal passage proceeds caudad, dorsal to the septum transversum. This passage 

 lies on a level with the gut and is at first hook-shaped, curving round the dorsal and medial 

 surface of the horn of the sinus venosus. As it passes back it comes to lie altogether on 

 the median side of the vitello-umbilical venous trunk, and is very much compressed later- 

 ally, the long axis of the lumen being dorsoventral. At the level where the vitello-umbilical 

 trunk is split into its components, the vitelline and umbilical veins, which level is also the 

 point of transition of these veins to the septum transversum from the body wall, the pleuro- 

 peritoneal passage becomes rounded and immediately opens into both the endoccelom and 

 exoccelom. The endoccelom and exoccelom are in wide communication for over one-third 

 the total length of the embryo, from the level of the posterior edge of the septum transver- 

 sum to the beginning of the belly stalk. 



At the level of origin of the belly stalk the splanchnopleure and somatopleure unite to 

 cut off communication between the endoccelom and exoccelom. The endoccelom lies to each 



