494 MISS K. M. PARKER ON THE 



Turning now to the facts revealed by the foregouig study of 

 Perameles, we find both the venae capitis medialis and lateralis 

 present. In Stage III. the first traces of the venous system 

 of the head are present in the form of isolated segments of the 

 vena capitis medialis. Further, in this stage, in the region of 

 the somites, there is on each side a series of dorsal offshoots from 

 the dorsal aorta (text-fig. 11) partially connected to form an as 

 yet incomplete longitudinal vessel lying, like the vena capitis 

 medialis, close against the neural tube. This vessel Hofi'mann 

 (7) described as representing the primordium of the posterior 

 cardinal vein. It is worthy of note that whilst the origin of this 

 vein from the dorsal aorta is thus clearly demonstrated, no 

 connection between the anterior segments of the vena capitis 

 medialis and the dorsal aorta could be observed, even after 

 careful study of the individual sections under the high power. 

 In the next stage (lY. P. nasuta 2 P), however, the vena capitis 

 medialis, though not forming a continuous longitudinal trunk, is 

 recognisable throughout the head region and is connected at 

 irregular intervals with the dorsal aorta. Furthermore, the 

 vena capitis medialis in this stage gives ofi" lateral capillaries 

 which anastomose to form the primordium of the vena capitis 

 lateralis. In the somitic region we find again a series of inter- 

 sec^mental ofishoots from the dorsal aorta. The vense capitis 

 lateralis and medialis continue to develop side by side, giving rise 

 to the condition shown in PI. II. fig. 7 {V.C.L. and V.C.M.). 

 (See also text-fig. 25.) In this stage the dorsal aorta and the 

 vena capitis medialis are connected by small capillaries (see text- 

 fio-, 20) whilst anteriorly the two vessels pass into continuity with 

 each other*. From the material available it is not possible to 

 say definitely how these connections arise, but the facts suggest 

 that the vena capitis medialis is derived from the dorsal aorta. 

 This view is further supported by the existence in the trunk 

 region of a longitudinal vessel which is undoubtedly formed from 

 a series of outgrowths from the dorsal aorta (text-fig. 11). This 

 vessel apparently bears the same relation to the posterior cardinal 

 that the vena capitis medialis does to the anterior cai'dinals, i. e. 

 it ^ives origin to capillaries which contribute to the formation of 

 the posterior cardinal. The orighi of the vena capitis medialis 

 from the dorsal aorta cannot, however, be regarded as proved, for 

 in the first stage in which it is recognisable, no connection with 

 the dorsal aorta could be traced ; in the two following stages (IV. 

 ■ and Y.) the connection is established and is lost in all subsequent 

 stages (e.g. YI.). 



From the descriptions of Salzer (17) and Grosser (6) it seems 

 that in the forms which they have studied, the vena capitis 

 medialis fuses in its entirety with the vena capitis lateralis, and 



* Professor Hatta tells me tliat he has found this condition also in the emhryo 

 of the lamprey. 



