744 PROFESSOR W. C. M'INTOSH 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, 

 PI. 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 Balfour made out a 

 special sheath, in fact, very soon after its formation (No. 11, p. 684). In Teleosteans 

 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 Esox (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, 

 PI. IV., become converted into the spacious chambers more or less rounded, especially 

 in the caudal region (PI. XV. fig. 4, nc), and subsequently into the more irregular 

 spaces (PI. XL fig. 11, and PI. XV. fig. 7, nc), 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 

 Kupffer describes, seem to take part in the process, nor do scattered yolk-spherules 

 (Balfour, 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 T. gurnardus on the fifth 

 day. Lereboullet 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, pi. ii. fig. 44). With its increase 

 in length the notochord grows in diameter, a condition which is precisely the opposite of 

 that described by Scott and Osborn in Triton, 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, i.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. 



