298 



LECTUPtE XII. 



Embrj'o Osseous Fish. 



The inner (mucous) layer of the globular yolk-sac sends from its 

 upper part a ctecal process forwards and another backwards, almost 

 co-extensive at first with the trunk above. But their growth is 

 checked by the adhesion of their blind ends to points of the serous 

 layer on the under part of the cephalic and caudal extensions of the 

 trunk : which points of adhesion become perforated, and establish the 

 mouth i^Jig. 79. d) and vent (ib. ^) : the intermediate mucous tract forms, 



at first, a short and straight 

 alimentai-y canal, commu- 

 nicating by a gradually 

 constricted aperture with 

 the remaining yolk-sac 

 (ib. d). In the Cyprinoid 

 fishes the vitellicle is ses- 

 sile ; i. e., it does not 

 hang,as a pedunculate sac, 

 from the exterior of the 

 body : the young Salmon 

 quits the ovum with the vitellicle in the form of a vascular oblong 

 appendage from the fore part of the abdomen. In both cases the 

 vitellicle is included, together with the intestinal canal, within the 

 parietes of the abdomen (ib. c), formed by the before-mentioned deve- 

 lopment and coalescence of the laminge ventrales. In the Tench the 

 yolk is divided only by the constricted communication with the 

 intestine, and is said to be 'internal ;' in the Salmon, and, also, in 

 Zoarces and Cottiis, the yolk is divided by a second constriction, 

 where it hangs from the ventral integuments ; the part within the 

 abdomen is called the ' internal yolk,' the part without is called the 

 ' external yolk.' 



The vascular channels, which are excavated in the soft embryonic 

 tissues of both the vertebral and visceral systems, and which convey, 

 at first, a plasma with coloui'less nucleated cells, unite into a longitu- 

 dinal sinus in the interspace of the two systems, where the aortic 

 trunk {ii) and the cardinal veins (m) are afterwards situated. This, 

 at first simple canal, receives or transmits vascular loops or arches 

 upwards to the laminae dorsales, and downwards to the laminte 

 ventrales ; those of the latter being most conspicuous that spread 

 over the vitellicle : it is here, also, that the red-colour of the circula- 

 ting cells, or blood-discs, is first perceived. The vitelline vessels in 

 osseous fishes are ramifications of a mesenteric vein, analogous to that 

 subsequently established to form the portal system of the liver. 



The heart is not developed from the longitudinal vessel in the 

 dorsal region of the abdomen. The larger branches, which ramify on 

 the vitellicle, unite into a trunk at its anterior part, which, being 



