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



In the myo-epicardium, with the exception of that of the sinus venosus, they are 

 extremely rare. In the sinus venosus, and particularly in those portions of the 

 mesothelium of the body-cavity and transverse septum which are directly con- 

 tinuous with the myo-epicardium, mitotic figures are comparatively numerous. 

 This finding would seem to indicate that the multiplication of cells of the myo- 

 epicardium of the heart takes place at this stage principally in the region of the 



sinus venosus. 



VEINS. 



VENA CARDINALIS ANTERIOR. 



The vena cardinalis anterior (plate 5, fig. 1) draws its blood principally from 

 the region of the brain. In Ingalls's embryo this vein begins at the junction of 

 two veins which course caudally from the region of the prosencephalon. Ingalls, 

 basing his interpretations upon the work of Mall 31 , regards the dorsal of these as 

 the source of the future sinus sagittalis superior ; the ventral one, the vena ophthal- 

 micus. In my specimen the region drained by the two above-mentioned veins 

 is occupied by a venous plexus. One tributary, which extends upward from the 

 region of the optic vesicle, may already be identified with reasonable certainty as 

 the ophthalmic vein. The much more extensive plexus above probably gives 

 rise to the embryonic superior sagittal sinus. The tributaries of the above- 

 described plexus come together medial to and behind the trigeminal ganglion, 

 where they form an enlarged venous sinus, between the trigeminal ganglion in 

 front and the acustico-facial ganglion behind. From this the anterior cardinal 

 vein passes caudally by two main channels, the dorsal of which is situated above 

 the origin of the acustico-facial ganglion, while the ventral one is placed medial 

 to it. Passing the interspace between the acustico-facial ganglion and otocyst, 

 these branches unite again medially to the otocyst, forming another enlarged 

 portion of the vein. Caudal to the otocyst it divides into three smaller veins, 

 which soon come together again. Opposite the first somite the anterior cardinal 

 vein becomes very small in diameter, but gradually becomes larger when traced still 

 farther caudally. From the second segment to its termination, the vena cardinalis 

 anterior is represented by two small veins which are closely related to one another. 

 In several places, however, they unite to form a single vessel. At the level of the 

 fifth body-segment the vena cardinalis anterior enters the vena cardinalis com- 

 munis (duct of Cuvier). 



In its course the vena cardinalis anterior receives tributaries on both its dorsal 

 and ventral walls. On the dorsal, the first is found in the region of the trigeminal 

 ganglion and proceeds from the direction of the mesencephalon to reach the 

 anterior cardinal vein at the posterior border of that ganglion. The second lies 

 just in front of the otocyst close to the rhombencephalon. The position which it 

 occupies (just in front of the otocyst) indicates that it is the same vein which 

 Mall 31 describes as the vena cerebri media. Behind the otocyst is a small stump 

 of a vessel which could not be traced far dorsally. Owing to its position, just 

 behind the otocyst, it becomes evident that this vein must be identical with the 

 one occupying a similar position in Ingalls's embryo and which Evans 8 has inter- 



