THE COD FAMILY. 239 



About the second day after hatching, the little cod have 

 grown stronger; they assume the normal position with the 

 yolk-sac ventral, and swim about in the water in straight lines. 

 During this day and the next the circulatory system extends 

 further and further towards the tail-end and the branchial 

 system grows more complicated (Plate IX, fig. 2). 



On the fourth day after hatching the yolk-sac has consider- 

 ably diminished, the dorsal fin arises mid-dorsally and the 

 integumentary vesicle (Ryder)^ develops. This is a large ex- 

 panded cap of cuticular tissue on the dorsal surface of the head, 

 and is a well-marked structure in all the gadoids and some 

 pleuronectids in confinement. 



By the 6th day the larvae have reached a length of about 

 ^ inch and the yolk-sac has almost disappeared, so that they 

 may be regarded as coming to an end of their larval career 

 about a week after hatching. At this stage, or soon after, they 

 appear to move into the mid-water and later to the bottom, 

 and it is rather difficult to develop them to a more advanced 

 stage in the ordinary tanks of concrete or wood. 



The latest stages in confinement showed complete absorption 

 of the yolk-sac. The head becomes greatly enlarged and the 

 lower angle of the jaw very prominent, with an upward slope of 

 the mandible. The cephalic vesicle gradually disappears and a 

 deep hollow occurs in the pre-maxillary region. The eyes are 

 large, deeply pigmented and have a silvery sheen. The change 

 in the character of the pigmentation from the four-barred 

 condition to a more diffuse type is clearly seen. The two 

 anterior bars gradually break up, being modified chiefly into a 

 mass of black pigment lying immediately dorsal to the abdomen. 

 The two posterior bars remain till later, but the dorsal parts of 

 one bar slowly coalesce with those of the other so that eventually 

 the post-larval form has a pair of longitudinal lines, dorsal and 

 ventral, with no trace of the transverse bars of the larval 

 form. Black chromatophores also occur on the head, the 

 angle of the jaw and on the ventral surface of the abdomen. 

 This stage in confinement did not exceed '175 inch in length, 

 but those at the same stage caught in the mid-water tow-nets 

 1 Ryder, Science, vii. 1886, pp. 26-29. 



