760 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



whereas the rounded form is only assumed later when the cells have increased in number, 

 and form two or three layers (No. 69, p. 103; cf. figs. 9, 12, Taf. iv.). Much later, when 

 the absorption of the yolk is accomplished, the lumen of the epiphysis becomes obliterated, 

 and it is separated off as an oval, lobed, or deeply folded solid mass of cells (No. 69, 

 p. 103, Taf. iv. fig. 17). 



The spinal cord, when fairly advanced, proceeds quite to the termination of the 

 notochord, but its general features call for no detailed description. Usually the terminal 

 filum is very delicate and attenuated ; but at times a remarkable enlargement is observed. 

 This final nervous swelling was very well seen in a young embryo of Motella mustela 

 (PI. XV. fig. 4, ne), but in other forms it was also made out, e.g., Cottus scorpius 

 (PL XIII. fig. 2) and Molva vulgaris (PL V. fig. 7). 



Auditory Organs. — The otocysts are very early differentiated — that is, about the 

 same time that the lens of the eye is invaginated and defined (as pointed out by Kupffer, 

 No. 88), i.e., in many pelagic forms about the fourth to the sixth day after fertilisa- 

 tion. In Salmo solar, according to Parker, the ears are pushed in from the outside 

 shortly before hatching; and he refers to these " auditory involutions " as "still widely 

 open" during his "first stage" (No. 117, p. 113). This description, however, is quite 

 unlike the mode of formation in the Teleosteans specially referred to in this paper; and 

 Lereboullet's account, in the case of Esox lucius, is certainly more in accordance with 

 observations at St Andrews, where he says that the early ears " are two small spheres, 

 symmetrically placed, and formed by the grouping of plastic elements, .... at first 

 solid; but becoming hollow, and transforming into the auditory capsules" (No. 93, 

 p. 529). The otocysts are, in fact, not involutions of the external epiblast, but solid 

 proliferations of the sensory or neurodermal epiblast (au, PL IV. figs. 4, 11, 16a). In 

 Lepidosteus Balfour and Parker describe the ear as originating from the under or 

 sensory layer, but as a hollow thickening, over which the epidermic layer is externally 

 continuous (No. 18); and Hoffman, while he rightly speaks of this external layer 

 as extending unbroken over the otocyst, says that the otocyst itself is formed as a 

 hollow invagination of the under-layer (Grundschicht), a condition not exhibited by 

 our sections of pelagic embryos. The earliest phase seems to be that of a rounded mass 

 just becoming visible in the early haddock, i.e., a solid proliferation of the sensory 

 stratum {conf. PL IV. fig. 11, the figure referred to, with Hoffman's, No. 69, figs. 3 

 and 4, Taf. i., and fig. 1, Taf. iv.), in which very soon a radial arrangement of cells can 

 be made out preparatory to the formation of a lumen. The latter rapidly appears 

 {au, PL IV. fig. 13 ; PL V. fig. 8), and is at first minute and spherical, but soon 

 enlarges to form a spacious ellipsoidal chamber [au, PL VI. figs. 5, 6), very obtusely 

 rounded, depressed laterally, and with its inner wall abutting against the neurochord (ne), 

 while on its outer side, and superiorly, it is separated from the exterior only by the 

 tegumentary epiblast (ep). The walls of the otocyst are very dense when the lumen is 

 .small (au, PL IV. fig. 13), but they apparently stretch as the chamber expands, and 

 become comparatively thin (au. PL V. fig. 9; PL VI. figs. 1, 2, 7). Lereboullet 



