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; ef: 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 
(Pl. XV. fig. 4, ne), but in other forms it was also made out, e.g., Cottus scorpius 
(Pl. XIII. fig. 2) and Molva vulgaris (PI. 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 Kuprrer, 
No. 88), 2.e., in many pelagic forms about the fourth to the sixth day after fertilisa- 
tion. In Salmo salar, 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 luctus, 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 (av, Pl. IV. figs. 4,11, 16a). In 
Lepidosteus Batrour and Parker describe the ear as originating from the under or 
sensory layer, but as a hollow thickening, over which the epidermiec layer is externally 
continuous (No. 18); and Horrman, 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, «e., a solid proliferation of the sensory 
stratum (conf: Pl. IV. fig. 11, the figure referred to, with Horrman’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 (aw, 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. VIL. figs. 1, 2, 7). LeresouLLer 
