DEVELOPMENT OF THE SKULL IN THE PIG. 



290 



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 hg-ht upon" embryos the youngest of which 

 were filming over this primordial "aqueduct;" the skin (cutis) is incomplete over 

 the top (Pkite XXVIII. figs. 1 & 2, au.), hut 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. I & 2, au.), 

 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, au.), the opening of the 

 aqueductus vestibuli is shown; but this is best seen in a horizontal section, viewed from 

 below (Plate XXIX. fig. 10), where it is seen imbedded in the periotic wall inside the 

 " tegmen tympani " {t.ty.) ; a little 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 ; I 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 iii. & iv. au.) ; 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 

 o-rnithic (compare Plate XXVIII. fig. 8, cl, and Plate XXIX. fig. 7, cL, with " Fowl's 

 Skull," Plate lxxxii. fig. 1, cl.). 



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 lie, as dense ensheathed masses of young 

 blood-corpuscles, in the most diffluent 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 XXVIII. 

 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 



* I have not been able to determine what relation this primary opening bears to the " aqueductus cochleae," 

 or whether it is related to it at all. 



2 R 2 



