DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 783 
heemal vessels. The fact, however, seems to be that the form-elements of the blood are 
for the most part derived from the periblast, the primary corpuscles alone being 
moulded apparently from the detached cells of the subnotochordal trunks. In those forms 
in which a vitelline circulation is developed, the removal of nucleated periblastie cells 
and the formation of sinuous lacunze (primary hzmal trunks) has been repeatedly 
observed, and may almost be taken as established. In those without such a yolk-circula- 
tion (and to them reference is in these pages chiefly made), the periblast also is seen in 
sections to break up into similar particles, and these doubtless pass into the sinus venosus, 
though in what way is not decided. Certainly the liver and alimentary canal, as well 
as the pericardial chamber itself, are, as already pointed out, in intimate relation to the 
periblast beneath the embryonic-trunk (PI. VII. figs. 1, 2, 6, 9; also Pl. XII. fig. 8), and 
the transmission of detached periblastic elements into the circulatory plasma may be 
accomplished without difficulty. Ryprr, in Salmo and Tyloswrus, found such corpuscles 
in the pericardial chamber (No. 141, p. 537). This further consideration favours the 
latter derivation rather than the subnotochordal origin, viz., the rapid decrease in the 
volume of the yolk, even in those which have no yolk-circulation. In such forms the 
yolk protrudes as a very bulky appendage (y, Pl. XIV. fig. 1), but shortly before, and 
especially after the blood-circulation is visible, it diminishes very rapidly (y, Pl. XVII. 
fiz. 1). Now, if before the hemal fluid flows through its proper channels, it were 
deriving its corpuscles from the yolk, and still more, if with the further development of 
blood-vessels in the trunk a corresponding increase in the number of corpuscles takes 
place, the rapid disappearance of the yolk is readily accounted for. It is noteworthy, 
too, that while the subnotochordal trunks are the first to be developed, the formation of 
the subintestinal vein and ceeliac artery quickly follows, and as these probably communi- 
cate with hepatic lacune, the periblastic elements would find easy entrance into the vascular 
system of the embryo. These nucleated cells, which make their way into the hemal 
plasma, are originally colourless, and LerEBouLLer describes them as at first spherical, 
afterwards becoming flattened and elongated. They rapidly acquire the characteristic 
tint. In weak and sickly embryos the circulation is languid and the corpuscles few, a 
feature LEREBOULLET also noted (No. 93, pp. 581-2). In monsters, especially double 
embryos, the circulation presents interesting features, each having its own circulation, 
though receiving nourishment from a common yolk. LEREBOULLET instances the case of 
a trout (double monster) in which the artery divides into two vitelline trunks, each of 
the two returning as veins to the corresponding embryo; while in another case of a 
double-headed embryo, which possessed two hearts, one alone received blood from the 
vitelline veins, the other heart received nothing (No. 94, p. 246). 
Renal Organs.—The differentiation of a renal tract takes place at a very early stage. 
We have seen that on each side of the notochord (PI. IV. fig. 10) cuboid masses of 
mesoblast are serially marked off as protovertebra (my) soon after the separation of the 
somatopleuric from the splanchnopleuric lamella. Just external to the protovertebre, a 
little distance behind the otocysts, a rod of cells is budded off from the splanchnopleure 
