894 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



like fine fibres, radiate over the whole area of the organ. The notochord transfixes the 

 tail considerably above the median line, leaving a much larger proportion of the lobe 

 beneath than above, and into this the blood-vessels pass. A little posterior to the 

 marked caudal diminution of the notochord the neurochord abruptly narrows, and a 

 delicate continuation (PI. XX. figs. 1 and 3) proceeds to the tip of the former. 



In the course of the succeeding month (February) the dorsal bend of the noto- 

 chord commences, so that on the 17th a considerable change in outline has occurred. 

 The upper border, especially along the posterior line, becomes prominent, while the 

 inferior is less so, and the hind edge assumes a broad blunt outline (PL XX. fig. 3). 

 It is evident that the inferior lobe of the tail in the early stage (PL XX. fig. 1) now 

 becomes more or less the posterior, a change, perhaps, partly due to the terminal and 

 upward curve of the notochord. The latter at this stage is much less marked than in the 

 salmon of the first da)^. The appearance of the hypural elements (PL XXVII. fig. 2), as 

 already indicated, probably aids in the transformation. The notochord is still less curved 

 than in the salmon of the first day, and the posterior hypural margin slopes downward 

 and forward. The hypural (cartilaginous) element most anterior (hue) is quadrate, and 

 devoid of the notch seen by Professor Huxley in that of Gastrosteus* Its longest side 

 is directed inferiorly and posteriorly. The posterior or superior hypural cartilage is 

 triangular in outline, its longest side being applied to the under surface of the ascending 

 notochord, which projects about half the length of this side beyond it. The bases of five 

 or six caudal rays rest on the larger hypural, and a somewhat smaller number on the 

 upper. They may be estimated at twelve in the earlier stage (March). In the following 

 month (April) the further curvature of the notochord upward is accompanied by a 

 tilting of the posterior edges of the hypurals into a nearly vertical position, and the 

 greatly elongated vessels now run straight outwards along the rays. The posterior 

 margin of the caudal fin has also become conspicuously crenate, and at this stage (PL 

 XXII. fig. 2) the inferior margin is more rounded than the superior, which ends after 

 a straight course somewhat abruptly in a crenation. In front of the tail, ventrally, a 

 slight inflection of the marginal fin occurs. The notochordal sheath now shows serial 

 constrictions indicating the separation of the centra. 



In the next stage (PL XXVII. fig. 3) the tail is considerably elongated, and its 

 vertical diameter is diminished. The notochord is less in proportion to the other parts, 

 while the anterior, or inferior, hypural has increased in length, and shows a distinct 

 upward curvature at the base. 



There are upwards of twenty caudal rays, i.e., about twenty-four, a larger number than 

 is present in the adult. Day records the number in the adult at fifteen to eighteen, 

 while in the St Andrews University Museum three specimens each possess twenty-one. 



The fin-rays show three vertical rows of articulations, and they spread out distally, 

 and terminate in fine fibres, like those of the embryonic fin. The marginal crenations, 

 posteriorly, are now so disposed that they correspond with the expanded ends just 



* Quart. Jour. Micr. Soc, 1859, p. 40, pi. iii. fig. 1. 



