496 MISS K. M. PARKER ON THE 



shows a double origin, the posterior cardinal vein is undoubtedly- 

 derived, but the details of the process of development of the 

 postcardinals cannot sa,tisfactorily be worked out in the material 

 available. The above account, however, agrees with that of 

 Evans (2) for the chick with regard to the origin of the post- 

 cardinal from capillaries derived partly from the Cuvierian duct 

 and partly from a vessel lying close to the neural tube. 



The vena capitis lateralis in Stage Y. is connected anteriorly 

 with the primary head capillaries arising from the first aortic 

 arch and also with groups of capillaries in the mandibular and 

 hyoid arches (see PI. II. figs. 6-8). 



In subsequent stages, the development follows the coui'se 

 described by Salzer (17) and by Grosser (5). Thus in a stage in 

 which twelve ci-anial nerves are established (Perameles nasufa, 

 13, vii. 05), the antei'ior cardinal vein runs medial to nerves Y. 

 and XII. and lateral to YII., IX., and X., and to the auditory 

 vesicle (text-fig. 25 B) ; i. e., the portion in the region of and 

 anterior to the trigeminal nerve and also that posterior to the 

 vagus, is derived from the original vena capitis medialis, the 

 intervening portion from the vena capitis lateralis. Traces of 

 the vena capitis medialis are, however, still present on the medial 

 side of nerves YII., IX., and X. 



Florence Sabin (16), in a recent note on the development of 

 cardinal veins in the chick, supports the viev/ that the cardinal 

 veins are derived from the dorsal aorta. She, however, states : 

 " The part of the head vein which lies close to the neural tube 

 arises from the arch of the aorta and is a part of the vascu.lar 

 system of the central nervous system ; the caudal part of the 

 head vein arises directly from the aorta." In this respect my 

 results difl'er somewhat from hers, for in Perameles the vena 

 capitis medialis {i. e. " the part of the head vein which lies close 

 to the neural tube ") is present before there is any trace of the 

 capillaries arising fi'om the arch of the aorta (Stage III.). It is 

 indeed secondarily connected with these, but as is shown in 

 PI. II. fig. 7 (V.C.M.) it also extends up to the extreme anterior 

 end of the head in close relation to the dorsal aorta with which, 

 in fact, it fuses. Since this vein exists before the formation of 

 the head capillaries which connect it with the arch of the aorta, 

 it obviously cannot be derived from that arch. 



In seems, therefore, that in Selachians (Hofimann (7)), the 

 chick (Evans (2), Florence Sabin (16)), and also in Perameles and 

 Ilacrojnis, there exists in the primary condition a. continuous 

 vessel lying close to the nerve-cord throughout its length. This 

 vessel is derived in the posterior and piobably also in the anterior 

 I'egion from the dorsal aorta. It contributes to the formation of 

 both anterior and posterior cardinal veins. 



It may be concluded that the presence in early stages of a vein 

 lying close against the neural tube throughout its length is 

 correlated with the relatively great importance of the central 



