512 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [o8] 



chorda cells enlarge they lose their lenticular form and gradually become 

 polyhedral, and but two to three of them are found side by side in any 

 one diameter of the chorda. As the growth of the tail of the embryo 

 proceeds, the chorda not only increases in diameter, but it also lengthens, 

 together with the other parts of the caudal extremity, and the metamor- 

 phosis from the solid condition to the vacuolated one proceeds from 

 before backwards to the slightly swollen caudal end of the chorda, where 

 it is still in connection with a caudal mass of undifferentiated cells, even 

 after great advances in its development have been made at its anterior 

 end, as shown in Fig. 34. The sheath of the chorda also becomes thin- 

 ner as development advances, and in Alosa, for exami^le, the sheath seems 

 to be formed mainly of the walls of the A^acuolated cells which come to 

 the surface. This view of the fate of the walls of the chorda cells lying 

 next the surface of the notochord, with their parietal nuclei, seems also 

 to be in accord with those of Gegenbaur and Balfour. The contents of 

 the vacuolated cells of the notochord in the embryos of osseous fishes 

 are not gelatinous, but quite fluid, and may for the most part be ab- 

 stracted by alcohol or glycerine, causing the chorda to collapse more or 

 less notably. 



The volume of the chorda as observed in the embryos of different 

 genera of the same relative age is subject to a very marked variation. 

 In i^roportion to the bulk of the remainder of the embryo it is most volu- 

 minous in the just hatched young of Alosa and Poniolobus. In other 

 families I have never met with it in anything like the same proportion- 

 ate size as compared with the other parts of the bodj'. In cross-section 

 in the Clupeoids it will measure quite three times as much proportion- 

 ally in area as in Gamhusia, Cyhium, Tylosnrus, Gadus, Pare])hi2)pus,Salmo, 

 Idus, JSsox, Morone, and Hippocamims. In the last-mentioned genus no 

 caudal fin is developed, and hence its terminal end undergoes no upward 

 flexure, as is the case with many other forms of Ganoids, Teleosts, and 

 Elasmobranchs. In Sip1iosto7na also there is apparently no alteration 

 in the direction of its caudal end, for here the five or six caudal-fin radii 

 are formed homocercally in the tail fold without affecting the direction 

 of the notochord. The tail of Hippocampus is prehensile, however, be- 

 fore the differentiation of the vertebral bodies, and while its skeletal 

 axis is still entirely notochordal. 



It would appear that in some forms, at least as development advances, 

 the vacuolated cells of the notochord divide and become smaller. Tliis is 

 apparently the case with Gamhusia pairueVis^ in which we may also note 

 a general acceleration in the development of tlie notochord, by which 

 it ])resents a tendency to form the external skeletogenous layer and be- 

 come constricted at i)oints corresponding to the muscle segments, to form 

 the vertebral bodies, even before the yelk-sack is absorbed. In the em- 

 bryos of this species we also find the cartilaginous sheath very thick at 

 period, and the constrictions, which are visible as the first indications 



