[93] EMBRYOGRAPIIY OF OSSEOUS FISHES. 547 



as iu Salmo, there is a subintestiual vein, which, unlike the sub-intesti- 

 nal vein of Salmo, is bent upwards just at the hinder extremity of the 

 yelk sack to end in the liver, in which it breaks up into a hepatic i)lexus 

 to emerge in the form of vitelline capillaries. 



In Gamhusia the subintestiual vein is short or absent. My reason 

 for thinking it absent is the fact that there is no vessel in this form 

 which has the same origin and termination in the liver as in the salmon. 

 What might be regarded as a subintestiual vein is the anterior end of 

 the caudal, which is bent downwards abruptly and traverses the poste- 

 rior portion of the abdominal cavity obliquely to divide on either hand 

 into a posterior vitelline vessel or vitelline canal on either side of the 

 yelk, which passes forwards to join and pass somewhat beyond the out- 

 going Cuvierian vessels into which the liver also pours its blood at one 

 side. From these lateral vascular arcs the vitelline capillaries take 

 their origin; they have a generally downward and forward direction. 



About the twenty-fifth day after impregnation, and five days after 

 hatching, in the series of cod embryos studied by the writer, there was 

 a complete circulation apparent in the branchial vessels, the aorta, and 

 cardinal veins, but only for a short way back. This primitive circula- 

 tion did not extend much beyond the extremity of the intestine or ab- 

 dominal cavity at this stage. In Fig. 40, twenty-two days after impreg- 

 nation no sign of circulation could be detected. In three days more, 

 however, blood corpuscles began to be more abundant and the vessels 

 could be seen to be slowly and progressively forming from before back- 

 wards iu the strand of vacuolating mesoblast underneath the notochord. 

 With the i)rogressive lengthening backwards of the aorta and caudal 

 vein, the point of union between them was also i^ushed backward, but 

 by what histological process was not made out. The point of union 

 between the caudal end of the aorta and the caudal vein is shown at p, 

 Fig. 45, representing an embryo ten days after hatching. A subintes- 

 tinal and lateral venous intestinal trunks were also developed at this 

 stage, which were joined together by short vertical vessels. 



24. — Development of the piaMENT cells of embryo fishes. 



In the embryo cod, as in young fishes generally, pigment cells begin 

 to be differentiated just under the epithelial layer of the epiblast at an 

 early period. In the cod they appear as small rounded scattered cells 

 of a slightly darker color than the surrounding tissues about the time 

 of the closure of the blastoderm. From that time forward, however, 

 they become progressively darker and more densely loaded with gran- 

 ules of melanin. They also soon lose their primitively rounded or bis- 

 cuit like form and become depressed and manifest a tendency to throw 

 out flattened pseudopods or prolongations in all directions. When far 

 advanced in develoi)ment, as in the later stages, the dark melanin gran- 

 ules do not entirely obscure the nucleus of the pigment-cell which may 

 be noticed in its center as a very refringent body entirely devoid of 



