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BULLETIN OF THE UNITED STATES FISH COMMISSION. 227 



lined, but the tail began to grow out before the embryo's body had em- 

 braced much more than one-fourth of the circumference of the vitellus. 

 On the third day the tail of the embryo had acquired considerable length, 

 and its free end was moved from side to side gracefully and rhythmic- 

 ally through the contents of the water-space. 



The water-space from the first was filled with an immense number of 

 free, refringent corpuscles, which made it difficult to make out the form 

 of the embryo during the early stages. These corpuscles were not of the 

 nature of blood-cells, and seemed to become less abundant towards the 

 close of the period of development within the egg. Nothing of a similar 

 character, as far as the writer is aware, has ever been encountered in the 

 water- space of any other Teleostean egg. So abundant are these cor- 

 puscles, at first, coupled with the opacity of the vitellus and the pecul- 

 iar whiteness of the germinal matter, that even an experienced ob- 

 server would be led to suppose at a first glance that all of the eggs were 

 bad, having the "rice-grain" appearance of blasted shad eggs. 



On the third day the vascular system begins to develop and the heart 

 to grow forward under the head, down over the anterior end of the yelk. 

 A pair of vascular arches (cuvierian ducts) are soon formed, just behind 

 the rudiments of the pectorals, which grow outward and split up into 

 vitelline capillaries and eventually join a median vitelline vessel which 

 empties into the venous end of the heart. The mouth is widely open 

 on the third day, and the branchial clefts are developed with a free cir- 

 culation through the arches. The caudal part of the aorta and caudal 

 vein is also developed at that time, and the intersegmental vessels are 

 developed a little later, with loops running out into the mesoblast of the 

 median natatory fold. 



The eyes of the embryos were unusually small for young fishes, and 

 reminded one during the early stages of the eyes of Ganoids and young 

 Amphibians. The choriod fissure was prolonged obliquely far forwards. 

 The eyes were pigmented on the fifth day. 



The air-bladder became perceptible on the tenth day, far forwards, 

 and as a dorsal outgrowth of the intestine, a little above and behind 

 the level of the insertion of the pectoral fins, and as it grew more capa- 

 cious the young fish commenced to swim higher in the aquarium. When 

 first hatched, on the sixth to the eighth day, the young exhibited a ten- 

 dency to bank up or school together like young salmon. They also, like 

 young salmon, tended to face or swim against the currents in the aqua- 

 rium, a habit common, in fact, to most young fishes recently hatched. 



The development of the fins was somewhat similar in general character 

 to that usually observed. On the second day the medial natatory fold 

 began to grow out on the dorsal and ventral side and the end of the 

 tail, but up to the fifth day no clearly-marked differentiation of any of 

 the unpaired fins had occurred. The first of the paired fins to appear were 

 the pectorals, which began to show themselves on either side of the 

 body on the third day, a little way behind the ear, as a pair of low Ion- 



