DEVELOPEMENT OF FISHES. 605 



as those ends approximate each other. The choroid appears in tlie 

 form of an inner cyhnder, applied to the snnlv back-part of the 

 lens, and its extremities, approximating and uniting, jn-odnce the 

 choroidal fissure : the eye is now the most conspicuous part of 

 the embryo, especially in the ovum of the Salmoiddm, and is a 

 useful sign to the pisciculturist of the impregnation and vitality 

 of the egg. 



The hinder cascal part of the intestine rapidly elongates, fi-om 

 behind forward, the yolk advancing in jjosition. The anterior 

 ciccum also elongates from Ijefore backward : the open part of 

 the intestine, which communicates with the vitelline sac, becomes 

 in the same measure constricted. 



When the two divisions of the heart arc bent upon one another, 

 the liver shows several small cfcca, which rapidly multijdy, and 

 become op)ake : it is situated, fig. 424, I, behind the heart and 

 above the yolk, now becoming reduced to a globule of oil, which 

 is long retained in the young Perch. 



The primordial kidneys appear as two p)arallel rows of rounded 

 cells, above the liver, their ducts uniting to form a tulje, which 

 runs above the intestine, and dilates above the hinder caical end 

 of the gut. 



The pectoral fins begin to bud forth : the protocercal mem- 

 branous fin-fold commences at the middle of the back, borders 

 the tail, and returns along the belly as far as the vitellus. Large 

 pigment-cells arc spread over the yolk-sac, which become stel- 

 late. IMuscular fibres appear in the myocommata as transparent 

 cylinders, without the transverse stria; : they move the tail 

 vigorously, and cause the embryo and its yolk-sac, in the Perch, 

 to rotate in the egg. This has increased in size by imbibition 

 of water, and its external coat is thinned by stretching; it now 

 gives way, and the embryo is extricated, about the tenth day in 

 the Pike and the twelfth day in the Perch. The size and shape of 

 the yolk-sac, fig. 424, c, vary in clltferent kinds of Osseous Fishes.' 



The vitelline vascular network, ib. d, is the first respiratory organ 

 of the fish : its divisions carry the blood-discs only in single files. 

 The outer tunic covering the vascular one permits the interchange 

 of gases between the blood and the water outside. This respired 

 or arterial blood is mixed with the venous blood which is returned 

 to the heart by the cardinal veins, and is distributed, so mixed, 

 by the arteries. The vitelline capillaries gi-adually exchange a 

 reticulate for a parallel longitudinal course, with diminution of 



' In artificial liatching, young trout, and especially char, show a diflBeulty iu extri- 

 cating the yolk-sac, and many perish from inability to liberate themselyes. 



