DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 745 



very thin, and at times almost imperceptible (nc, PL VII. fig. 6). The true notochordal 

 sheath during the later larval periods is very delicate and fine (PI. VII. figs. 6, 6a), nor 

 does it, as Balfour indicates (No. 11, p. 546), ever become thicker or more definite. It 

 is nucleated (n, PI. XV. fig. 7), as Gegenbaur showed in the greatly thickened sheath 

 of Salmo salar ;* but the nuclei are irregularly arranged, and in some sections they 

 are so sparse as to suggest the presence of an enucleate stratum (cs, PI. XV. fig. 7), 

 though this condition is easily explained by its mode of origin. In horizontal sections 

 of the chorda the flattened cells of the sheath overlap and produce a more or less regular 

 tessellated appearance (Pi. XV. fig. 7). Kupffer says that this sheath in the herring is 

 homogeneous and without nuclei (No. 87, p. 222), while he describes a round nucleus in 

 each chamber of the vacuolated notochord. Such nuclei in our forms are rare, though 

 sections often pass through the points, where several septa unite and produce the same 

 appearance as nuclei in the septa would do, but they are simply sections of the junction 

 of cell-wells, or, at times, merely the collapsed contents of the notochordal chambers. 



Of the subnotochordal rod, which has been described by Balfour, Oellacher, Ryder, 

 and others, nothing definite can be here stated. It would, in fact, not appear to be 

 developed in the forms specially considered in this paper, though Ryder mentions it in 

 Alosa and Salmo as a well-marked strand of cells ; and Oellacher is of opinion that it 

 shares in the development of the aorta along the under surface of the chorda dorsalis. 

 The intruding mesobiast limiting the chorda below in the Gadoids, gurnard, and others 

 is an indefinite lamella figured in its earliest condition in PI. IV. figs. 12 and 18, which 

 subsequently forms a median meshwork in which the early haemal lacunae are developed, 

 while laterally the renal connective and other tissues are formed out of it [vide PI. VII. 

 figs. 1, 4, 6).t 



Vertebral Column. — The vertebral column and its costal appendages belong as such 

 to a stage subsequent to the larval condition proper, and, in this place, little more 

 can be done than simply to touch upon certain points observed before the close of the 

 first month after extrusion. 



The cod and haddock will be mainly referred to, as the condition of the vertebral 

 column shows great differences in various Teleosteans ; in some forms cartilage-cells 

 appearing, and cartilaginous arches developing soon after hatching, whereas in others 

 no such elements are present until the embryo is about a month old. Lereboullet, 

 indeed, was unable to make out any ossification in the perichordal sheath, in Perca, until 

 the young fish was three months old (No. 93, p. 644). 



The condition of the notochord before and after hatching has been described, 

 and sections of Q. morrhua or G. aglejinus, on the seventeenth to the twentieth day, 

 show the same simple structure almost unchanged — the cuticular layer or nucleated chordal 



* Comp. Anat., Lond. 1878, fig. 2216, p. 427. 



t The myotomes are broken up into fibres about the ninth day (two days before hatching in P. flesus). Eight or 

 ten of these fibres, in horizontal section, are seen passing across the shorter axis of the myotome, which is rectangular, 

 and measures about - 001 in. x "0018 in., the longer measurement including the columnar external stratum of cells 

 lying beneath the epiblast. 



VOL. XXXV. PART III. (NO. 19). 6 C 



