PEVELOPEMENT OF FISHES. G05 



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

 form of an inner cylinder, applied to the sunk back-part of the 

 lens, and its extremities, approximating and uniting, produce the 

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

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

 useful sign to the pisciculturist of the impregnation and vitality 

 of the egg. 



The hinder ca^cal part of the intestine rapidly elongates, from 

 behind forward, the yolk advancing in position. The anterior 

 caecum also elongates from before 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 are bent upon one another, 

 the liver shows several small creca, which rapidly multiply, and 

 become opake : it is situated, fig. 424, /, 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 parallel rows of rounded 

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

 runs above the intestine, and dilates above the hinder crecal 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 are spread over the yolk-sac, which become stel- 

 late. Muscular fibres appear in the myocommata as transparent 

 cylinders, without the transverse stria3 : 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 different kinds of Osseous Fishes. 1 

 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 gradually exchange a 

 reticulate for a parallel longitudinal course, with diminution of 



1 In artificial hatching, young trout, and especially char, show a difficulty in extri- 

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



