of the Fishery Board for Scotland, 



213 



disintegrating the periblast. It probably also subserves a respiratory- 

 function. It almost completely disappears on hatching (&.s., fig. 3). 



The circulating corpuscles are used up at the growing parts, and very 

 few return to the yolk sac until after hatching, when the yolk sac becomes 

 simply a wide vessel conducting the corpuscles to the heart from the caudal 

 vein. When the larva hatches there is not so great a demand made upon 

 the yolk and the tissues, and there is, therefore, a copious supply of yolk 

 corpuscles circulating in the vessels ; this is especially well seen in the 

 eggs of Murcenidce. At hatching the vertebral vein {v. v., fig. 27) 

 has appeared, and also the ductus Cuvieri {d.c.y figs. 3 and 27). Large 

 numbers of corpuscles enter the sinus venosus (anterior portion of the yolk 

 sac) {s.v., fig. 3) by the ductus Cuvieri and from the posterior portion of 

 the yolk sac. 



In an egg measuring '94 mm. diameter, and having an oil globule 

 (posterior), a stream of small granules {gr.) resembling the nuclei in the 

 periblast was seen passing from the caudal vein {c.v.) into the yolk sac 

 (fig. 7). 



Circulatory System. 



At the time of hatching, the system of vessels in which the yolk 

 corpuscles circulate is as follows : — The heart opens by its venous end 

 into the yolk sac. From the heart the corpuscle passes into the first aortic 

 arch (a.a., fig. 1), whence it proceeds to the lateral artery fig. 1) — 

 the forerunner of the branchial vein. The two lateral arteries unite to 

 form the dorsal aorta (d.a., figs. 1 and 15), by which the corpuscle is 

 conducted to a point a little short of the extremity of the tail. It is 

 brought anteriorly again by the caudal vein (c.v., fig. 25), which lies along 

 the base of the anal marginal fin. The caudal vein crosses the rectum 

 and debouches into the posterior end of the yolk sac (p.e., fig. 25). In 

 the head the circulation, as far as was made out, is as follows : — From 

 the lateral artery the corpuscle passes into the carotid artery (c.a.), and 

 from the latter passes by the ocular artery (o.a.) to a lacuna (l.) round the 

 lens of the eye (figs. 1 and 7). The continuation of the carotid 

 anteriorly was not made out. From the lacuna round the lens the corp- 

 uscle returns by the ocular vein (o.v., fig. 1) to the jugular vein (j.v., figs. 1 

 and 7), which may be single (fig. 7) or double (fig. 1). Where the jugular 

 vein is single it divides into two branches (v.a. and a^., fig. 7), which 

 open, one on each side, into the yolk sac at a point ventral to the ear 

 capsules. Where there are two jugular veins (fig. 1), they open, one on 

 each side, into the yolk sac in the same region. 



The yolk sac acts simply as a large vessel. 



There is thus in the pelagic egg a circulatory system exactly homologoub 

 to the circulatory system in a demersal egg; the only difference is in the* 

 arrangement of the system in the yolk sac. In the pelagic egg the yolk 

 sac is one large vessel ; in the demersal egg it is broken up into a network 

 of vessels. 



The Respiration of the Teleostean Embryo. 



A comparatively elaborate respiratory system is developed in the 

 teleostean embryo. The main organ of respiration in the embryo is the 

 gut, or, more correctly, the delicate embryonic canal which is the forerunner 

 of the gut proper. Where the term ''gut" is used in this paper, this 

 rudimentary gut is referred to. 



