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 (Pl. 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 (PI. 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 day. ‘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 (uc) is quadrate, and 
devoid of the notch seen by Professor Huxuey 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 (PI. 
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, 7.c., 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, pl. ili. fig. 1. 
