DEVELOPMENT OF THE SKULL IN THE PIG. 299 
dermal layer to form the ear-sac, the cavity of which was wide open on the outer surface. 
In this piece of work “it was my hap to light upon” embryos the youngest of which 
were filming over this primordial ‘aqueduct ;” the skin (cutis) is incomplete over 
the top (Plate XXVIII. figs. 1 & 2, au.), but the passage itself, leading into the rudi- 
mentary labyrinth, is closed by a gelatinous plug. The periotic walls come as near to 
cartilage, in their commenced solidification, as the investing mass and facial arches, and 
the outline of the sacs can be fairly made out. Their general form can be seen by 
referring to the horizontal views (Plate XXVIII. figs. 5 & 8, and Plate XXIX. fig. 7> 
au.); but they are well outlined on the external surface (Plate XXVIII. figs. 1 & 2, aw.), 
and they are seen to be tuberose bodies, having a straightish inner margin, a sublobular 
outer margin, and their broadest end behind. ‘They are separated by the breadth of 
the investing mass with its enclosed notochord, and this tract is narrowest in front. 
When the upper face is slightly pared off (Plate XXIX. fig. 6, aw.), the opening of the 
aqueductus vestibuli is shown; but this is best seen in a horizontal section, viewed from 
below (Plate X XIX. fig. 10), where it is seen imbedded in the periotic wall inside the 
**tegmen tympani” (¢.ty.); alittle behind it is seen the portio dura, which forms by its 
boring the “ aqueductus Fallopii.” In the same figure the opposite side of the section 
was made lower down, so that the roof of the tympanum (tegmen) is cut away, and 
the tympanic cavity cut through, exposing the head of the second postoral arch and the 
“aqueductus ” just above its entrance into the cavity of the ear-sac*. 
The reader will observe that this passage has the appearance of being double; TI could 
not, however, find more than one perforation. This opening into the auditory sac, which 
is large in my first and second stages in the Frog, has closed in the third stage (‘+ Frog’s 
Skull,” Plates mr. & tv. av.); in the Salmon (‘*Salmon’s Skull,’ Plate v.) it has not 
closed in “ fry” of the first summer. 
As for the cavity of the ear-sac, it is at present very rudimentary; the canals are but 
beginning to bud out from the postero-superior region, and the cochlea is perfectly 
ornithic (compare Plate XXVIII. fig. 8, ¢/., and Plate XXIX. fig. 7, c/., with “ Fowl’s 
Skull,” Plate Lxxxu. fig. 1, e/.). 
The sections show the larger nerves and vessels, which serve as excellent landmarks, 
especially the trigeminal, the portio dura, the glosso-pharyngeal, the vagus, and the 
hypoglossal nerves (5, 7%, 8, 8%, 9). The three last nerves all pass through soft stroma 
in the angle between the auditory sac and the investing mass; the large vessels also 
(‘basilar artery” and “internal carotid”) all he, as dense ensheathed masses of young 
blood-corpuscles, in the most difuent stroma, the fluidness and instability of which makes 
it an admirable “ soil” for these fast-growing countless * roots.” Before passing to the 
next stage I must again refer the reader to the diagrammatic figure (Plate XXVIIT. 
fig. 5), that he may compare it with what I have already described in the embryos of 
the Fowl, Frog, and Osseous Fish at a similar stage. With the vantage-ground of this 
* T have not been able to determine what relation this primary opening bears to the “ aqueductus cochle,” 
or whether it is related to it at all. 
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