YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 25 



It will be seen from the foregoing thai while the exact stage and degree of development 

 of the various parts of the alimentary system in these twin embryos are not the same, yet 

 in general they agree perfectly, showing the same plan of structure, and each helps to explain 

 appearances found in the other. Taken together, they give valuable information as to 

 the stage of the gut in the 18-19 somite stage of development, especially in regard to the 

 gill pouches and the cloaca. 



THE CIRCULATORY SYSTEM. 

 THE HEART. 



The heart occupies the whole of the body ventral to the pharynx, and between the 

 stomodaeum in front and the septum transversum and yolk sac behind. This region is 

 very prominent and bulging and the chambers of the heart are indicated on the outside by 

 distinct protuberances. The pericardial cavity (plate 1) is very spacious and in the sec- 

 tions appears triangular, with walls nearly equal and angles all well-rounded, with a heart 

 chamber lying in each angle, the atrium and atrioventricular canal dorsally and to the left, 

 the ventricle ventrally, and the bulbus cordis to the right dorsally. The heart is still in the 

 form of a simple tube, but is very strongly S-shaped, with abrupt flexures. At two of these 

 flexures are the constrictions of the tube which mark the limits of the various chambers, 

 visible both on the inside and outside of the muscular tube and also in the endothelial tube. 



The muscular wall of the heart (plate 3, fig. 4; plate 4, figs. 1 and 2) is several layers 

 thick, and forms a large, wide tube. The endothelium is 1 layer thick and forms a very 

 small tube (plate 4, figs. 3 and 4) lying in the very center of the muscular tube and every- 

 where separated from the muscle by a considerable interval, which is entirely devoid of 

 nuclei but shows many fine fibrillar processes and appears to contain a very clear, homo- 

 geneous matrix. In the sinus venosus and the atrium, however, the endothelium is closely 

 applied to the muscular wall. Mall (1912), in an article on the development of the heart, 

 states that that part of the tube forming the atrium can be very early identified by this 

 disposition of the endothelium, the apposition of the latter to the muscle being complete 

 in an embryo of 3.5 mm., while in the rest of the heart tube they are widely separated. The 

 contact is complete here also, and there is an abrupt change at the atrial canal, where the 

 endothelial tube becomes quite narrow and lies free from the muscle. 



The heart as a whole resembles very closely that of embryo Pfannenstiel III, modeled 

 by Low (1908), except that it possesses a much larger bulbus cordis, resembling in this 

 respect the Meyer embryo modeled by Thompson (1907). Mall (1912) shows an illustra- 

 tion of the heart of an embryo of 3.5 mm. very similar in shape and appearance (both of 

 the muscular and endothelial tubes) to the one here described. 



The duct of Cuvier on each side passes into the septum transversum just lateral to 

 the pleuroperitoneal passage (plate 1 ; plate 3, fig. 4; and plate 4, fig. 2) and is immediately 

 joined by the immense trunk of the united umbilical and vitelline veins of each side, to 

 form the horns of the sinus venosus. From here the main portion of the sinus venosus 

 runs transversely, embedded in the septum transversum and joining the two horns. It is 

 convex forward and its most forward portion issues from the septum, and has a very short 

 piece of the dorsal mesocardium (plate 4, fig. 2) suspending it to the roof of the pericardial 

 cavity. This ends at the beginning of the atrium. The atrium consists of a large chamber 

 slightly subdivided on its surface by shallow dorsal and ventral longitudinal grooves, which 

 unite at the anterior border. The atrial canal lies to the left, while the atrium runs forward 

 a short distance in the median line, to end between the ventricle and bulbus cordis. 



