A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OK SOMITES. 163 



SEPTUM TRANSVERSUM. 



The septum transversum is a thick plate of mesenchyma which is lined on 

 either side with the mesothelium of the ccelomic cavity. It divides off the peri- 

 cardial from the pleuro-peritoneal cavity, except posteriorly and laterally, where it is 

 incomplete. It is placed across the body at the level of the venous end of the 

 heart and liver diverticulum and is directed obliquely from in front dorsally and 

 cephalad. As near as can be determined, it is placed opposite the fourth and 

 fifth body-somites. With reference to Mall's 34 schema (Mall's fig. 400), it may be 

 said that in position the transverse septum of my embryo lies dorsally at a corre- 

 sponding position which he has shown for embryos of 2 and 4 mm., and is directed 

 at an angle which would pass some place between those of the 2 and 4 mm. embryos. 



Laterally and in front the transverse septum is attached to the body-wall. 

 Postero-medially it is attached to the posterior body-wall by means of the sinus 

 venosus and fore-gut. Postero-laterally its border is free and covered with 

 mesothelium. On either side, between this border and the posterior body-wall, 

 are the communications between the pericardial and pleuro-peritoneal cavities. 



Within the substance of the transverse septum are found the following 

 structures: Laterally, running along its attachments to the body-wall, are the 

 umbilical veins. Extending from below, nearer the median plane, are the vitelline 

 veins. The network formed at the terminal ends of these veins and the sinus 

 venosus formed by their union all lie within the transverse septum. In addition 

 is the relatively large hepatic diverticulum, which occupies a central position 

 within the septum. 



Structurally the transverse septum is composed of a dense syncytium of closely 

 packed mesenchymal cells. At the time of fixation these cells were undoubtedly in 

 a state of rapid growth, for mitotic figures are very abundant. 



EMBRYONIC MEMBRANES. 



AMNION. 



The amnion forms a closed sac, in which lies the greater part of the embryo. 

 It is composed of two layers of epithelium: an inner, directly continuous with the 

 skin ectoderm, and an outer, directly continuous with the mesothelial lining of the 

 coelom. Between these layers is a small amount of mesenchyma. 



LINE OF REFLECTION. 



The amnion is reflected from the body of the embryo as follows: cephalad 

 it gains attachment to the body-wall at the lower border of the pericardium; 

 laterally the line of reflection crosses the yolk-stalk on either side and continues 

 directly caudally along the edge of the body-wall to the body-stalk. Although 

 directly continuous with it, the demarcation between the anterior body-wall and 

 the amnion is clearly distinguishable because of the greater thickness of the 

 former. The body-wall ends abruptly not only by a sudden dimunition of its 

 size, but also by giving origin to the villus-like projections described above (plate 

 1, fig. 2). LTpon reaching the body-stalk the two lines of reflection of the amnion 



