744 PROFESSOR W. GC. M‘SINTOSH AND MR E. E. PRINCE ON 
they become flattened. A simple series of these transverse divisions, termed by him 
“ secondary cells,” is formed, and in a longitudinal section of the notochord he figures the 
various stages (No. 87, Taf. iv. fig. 44). The irregular transverse septa in section (nc, 
Pl. IV. fig. 12), are, however, evidently due to the adhesion of the walls of the primary 
notochordal cells, and the confluence of their protoplasm to form large interstices.* 
These septa become still more desiccated, and form a fine but complex meshwork, the 
outermost portions of which constitute a limiting membrane. No such investment as the 
latter as yet exists, though at a very early stage in Elasmobranchs BAaLFour made out a 
special sheath, in fact, very soon after its formation (No. 11, p. 684). In Teleasteans 
the neurochord above, and the hypoblast beneath, are in direct contact with the 
constituent cells of the notochord during the early metamorphosis just described. A 
stratum of flattened mesoblastic cells, it is true, at so early a stage as fig. 12, Pl. IV., may 
clothe the sides of the chorda, nc, while a thickened layer of similar cells may intrude 
between it and the hypoblastic enteron, g, destined, no doubt, to contribute to the later 
perichordal sheath. This external mesoblast is probably the special sheath described by 
LEREBOULLET at an early phase in Hsox (No. 93, p. 527); but at this stage the mass of 
cells is external to and independent of the notochord, which must be regarded as a naked 
cord of cells undergoing rapid vacuolation. 
When vacuolation has proceeded so far that the mere transverse fissures of fig. 12, 
Pl. IV., become converted into the spacious chambers more or less rounded, especially 
in the caudal region (Pl. XV. fig. 4, ne), and subsequently into the more irregular 
spaces (Pl. XI. fig. 11, and Pl. XV. fig. 7, ne), those collapsed cells which are not 
included in the septa will be pushed outwards, and form, as in fact they do, a continuous 
circumscribing sheath. The process is purely one of vacuolation, and the breaking down 
of the boundaries of smaller cells to form larger ones. No dot-like aggregations, such as 
Kuprrer describes, seem to take part in the process, nor do scattered yolk-spherules 
(Baxrour, No. 11, p. 684) or oily elements occur (LEREBOULLET, No. 93, p. 527) in the con- 
tents of the notochordal cells. The contents of the cells are fluid, clear, and homogeneous, 
and often exhibit a slightly pinkish tint in certain lights, as in 7. gurnardus on the fifth 
day. LereBouLer did not notice in his forms the early condition—the primary cells of 
the notochord, for it was already transversely striated when he first observed its 
structure; and he notes the remarkable feature, just referred to, that through all its 
substance oily elements are dispersed (No. 93, p. 527, pl. ii. fig. 44). With its increase 
in length the notochord grows in diameter, a condition which is precisely the opposite of 
that described by Scorr and Oszorn in Z’riton, for in that form the notochord is largest 
in cross-section during its earliest stages, and greatly diminishes in diameter during sub- 
sequent stages (No. 147, p. 467). The increase in diameter of the Teleostean notochord 
stretches the cells of the sheath, z,e., the superficial cells of the chorda; thus they become 
* These interstices, with fine membranous limits, form a series of discs placed one behind the other along the 
whole length of the chorda. They form large discoidal cells, which at many points do not entirely pass across the 
notochord, as they vary considerably in diameter. 
