DEVKLOPMENT OF THE UEAKT IN MAltSUPIALS. 493 



form of curvature, the right pviuioi'dium being larger and more 

 markedly curved than the left. This asymmetry occurs to a 

 somewhat less extent in the Oiiychoyale embryo of this stage, 

 and also in a, number of embryos of Dasyurus viverrinas of about 

 the same stage, so that evidently at this period the right and 

 left primordia of the heart develop indepen<leutly of each other. 

 It may be suggested that the greater length of the i-ight heart- 

 tube is to be accounted for by the fact that it is destined to 

 form the convex, longer side of the completed ventricular 

 limb. 



B. Development of the Cardinal Veins. 



Before summarising the I'esults of my observations on the 

 development of the cardinal veins, more particularly the anterior 

 cardinals, it may be useful to give a short resume of previous 

 work in this field. 



Hofimann (7) in 1893 described the development of both 

 anterior and posterior cardinals in Selachians by the formation 

 of a series of offshoots from the dorsal aorta?. These offshoots 

 become connected on each side to form a continuous longitudinal 

 trunk. He figures capillaries lying on both medial and lateral 

 sides of the auditory vesicle, but makes no comment thereon. 



Salzer (17) in 1895 described the development of the anterior 

 cardinal veins in the guinea-pig. According to him, the first 

 vein of the head arises on the medial side of the cranial ganglionic 

 primordia. A vein next arises lying latera.1 to the ganglionic 

 primordia of nerves VII., IX. & X. and to the auditory vesicle. 

 This vein, which Salzer calls " vena capitis lateralis," communi- 

 cates with the medial vessel and seems to be formed fi-om a series 

 of lateral outgrowths from it. The medial vessel degenerates in 

 the region of nerves VII. to X., so that for a time there is a 

 condition in which the vein of the head runs medially to the 

 trigeminal nerve, then, passing laterally, runs outside nerves VII., 

 IX. & X. and the auditory vesicle, and finally passes round the 

 medial side of nerve XII. before opening into the Cuvierian duct. 

 In subsequent stages, the process of development of the lateral 

 trunk is continued anteriorly and postei-iorly in the i/egion of 

 the trigeminal and hypoglossal. Thus the definitive anterior 

 cardinal vein runs laterally to all the cranial nerve-roots. 



Grosser (6) in 1907 gave a similar description of the develop- 

 ment of the anterior cardinals throughout the vertebrate series. 

 He calls the vein lying medial to the nerve-roots the ve7ia capitis 

 medialis, and the lateral vessel the vena capitis lateralis. The 

 former develops first and lies close against the neural tube. 

 From it are given off lateral vessels which become connected on 

 the outer side of the nerve-roots to form the vena capitis lateralis. 

 The vena capitis medialis persists only at its anterior end, the 

 rest of the anterior cardinal being derived from the vena capitis 

 lateralis. 



34* 



