A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 161 



represent the caudal arteries in their earliest stage, but as yet they can not be 

 said to exist as definite arteries. I have referred to one of them in plate 5, figure 2, 

 as the "arterial plexus of the tail." 



Arteri(v Umbilicales. The umbilical arteries are formed by the union of the 

 lower ventral segmental arteries, including all those caudal to the twentieth seg- 

 ment. Apparently one or two small capillary twigs from the tail plexus also 

 enters into its formation. Anastomoses between the individual roots of the artery 

 are apparent, thus giving rise to a network. In plate 5, figure 2, the network is 

 represented diagrammatically, since I found it unprofitable to attempt to plot 

 them with any degree of accuracy. In the first part of their course, where they 

 lie on either side of the allantoic duct, the umbilical arteries are small and indis- 

 tinct. J-'oon, however, they rapidly enlarge and fuse together to form a large 

 trunk within the body-stalk. Reaching the chorion, the single umbilical artery 

 breaks up into a number of branches, which, after repeated division, terminate 

 as capillaries in the substance of the villi. The relation of the allantoic duct to 

 the fork formed by the fusing umbilical arteries has been described above. 



CCELOM. 



The ccelom is represented by a continuous elongated cavity which is in wide 

 communication with the extra-embryonic co?lom. Already it can be divided into 

 two distinct parts, the pericardial and the pleuro-peritoneal cavities. The first 

 of these surrounds the heart, except where the heart is attached by means of its 

 sinus venosus behind and by its truncus arteriosus above. 



PERICARDIAL CAVITY. 



The pericardial cavity reaches its highest point in the region of the bulbus 

 cordis. Its form is shown in plate 6, figures 3 and 4. It is bounded dorsally by 

 the pharynx and septum transversum, ventrally and laterally by the thin body- 

 wall. Its floor is formed by the septum transversum. The floor is deficient on 

 either side dorsally where the transverse septum is not yet complete, and in the 

 space between it and the posterior body-wall the pericardial cavity establishes 

 its communication on either side with the pleuro-peritoneal cavity which lies below. 



PLEURO PERITONEAL CAVITY. 



The pleuro-peritoneal cavity is divisible into two portions, an upper and a 

 lower. The upper is formed by two narrow limbs which unite below the yolk- 

 stalk to form the single lower portion. The limbs of the upper portion join the 

 pericardial cavity high up on its dorsal surface. At first they are directed dorsally 

 and caudalward, but very soon bend directly caudal ward. As viewed from 

 behind, their median borders are not straight, but each presents two curves, the 

 concavities of which are directed medially. Fitting into the spaces formed by these 

 curves, as shown in plate 6, figure 4, is the lung diverticulum above and the hepatic 

 diverticulum below. That portion of the digestive tube which I have considered 

 as the gastric region lies opposite the constricted area between the two enlarged 

 spaces. 



