DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 783 



haemal 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 periblastic cells 

 and the formation of sinuous lacunae (primary haemal 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. Eyder, in Salmo and Tylosurus, 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, PI. XIV. fig. 1), but shortly before, and 

 especially after the blood-circulation is visible, it diminishes very rapidly (y, PL XVII. 

 fig. 1). Now, if before the haemal 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 cceliac artery quickly follows, and as these probably communi- 

 cate with hepatic lacunae, the periblastic elements would find easy entrance into the vascular 

 system of the embryo. These nucleated cells, which make their way into the haemal 

 plasma, are originally colourless, and Lereboullet 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 (PL IV. fig. 10) cuboid masses of 

 mesoblast are serially marked off as protovertebrae (my) soon after the separation of the 

 somatopleuric from the splanchnopleuric lamella. Just external to the protovertebrae, a 

 little distance behind the otocysts, a rod of cells is budded off from the splanchnopleure 



