718 
PROFESSOR T. J. PARKER OK THE 
confirmation of the lateral fin theory. Balfour himself considered that, if they could 
be found in the embryo or in some less specialized form than the skate, their presence 
would furnish an argument in favour of the theory. In quoting this opinion of 
Balfour’s (22, p. 232) I have pointed out that the vein in question is now known to 
exist in at least four genera of Selachians, and that in an embryo of Scymnus licliia , 
corresponding very nearly with Balfour’s “Stage O” (1, Plate VIII., figs. 0, O'; 
22, Plate XXXII., fig. 13), it is a large and important trunk. Figs. 29-32 (Plate 37) 
show the relations of the lateral vein in various transverse sections of this embryo; 
from its conspicuous character one is rather surprised to find that Balfour makes no 
mention of it in any of the forms investigated by him. Mr. Sedgwick, however, 
informs me that a re-examination of some of his sections has shown the vein to be 
present. 
In the paper last referred to (22) I have also hazarded the suggestion that the 
lateral veins may be genetically connected with the lateral vessels of a vermian 
ancestor. As Balfour has shown, the dorsal aorta is the primary dorsal trunk of the 
vertebrata, the sub-intestinal vein their primary ventral trunk, the cardinal veins being 
quite secondary structures. I have, unfortunately, no Selachian embryos of an earlier 
stage than “ 0,” and am therefore unable to say at what period the lateral vein makes 
its appearance, whether before or after the cardinals. If further investigation should 
show it to arise before the latter it will have to be definitely reckoned as one of the 
primary vascular trunks of the Protochordata; if shown to have a later origin its 
phylogenetic significance will be left still uncertain. 
In any case there can be no doubt that the blood-vessels must not be left out of 
consideration in instituting comparisons between the Vertebrata and the lower types. 
For instance, according to Hubrecht’s hypothesis (9), the Chordata are derived from 
Nemertean-like forms, in which the lateral nerve-cords have coalesced dorsad of the 
proboscidean sheath, which latter has been modified into the notochord. Comparing 
the vascular trunks of a Nemertean with those of an embryo Selachian, we find in 
both a dorsal and a pair of lateral vessels ; the lacunae described by Oudemans (18), 
in Valencinia, as lying symmetrically ventro-laterad of the proboscidean sheath may 
also, perhaps, be compared with the cardinal veins, but the ventral vessel, so charac¬ 
teristic of vertebrates, is wholly absent in the worm. 
Again, if with Dohrn, Semper, and others, we compare a vertebrate with an 
inverted Annelid, we find certain interesting agreements between the blood-vessels of 
the two. In Chaetopods there is a dorsal vessel in which the blood flows forwards 
(from tail to head), and which is intimately connected with the dorsal wall of the 
intestine. This, on the inverted Annelid theory, answers to the sub-intestinal vein of 
the Selachian embryo, in which the blood also flows forwards, and which arises in the 
splanchnopleure. The dorsal aorta would then correspond with the supra-neural 
vessel of the worm, and the lateral vessels of the two types would agree with one 
another ; the myelonal vein of Selachians might also be compared with the sub-neural 
