DEVELOPMENT OF THE SKULL IN THE PIO. 



303 



part of the rod, is continuous with the long descending part of the arch (see Plate XXX. 

 fig. 9,sf.h.); it is the head of the stylohyal. Here we see that the second postoral 

 arch grafts its capitular portion on to the auditory segment, and splits its tubercular 

 portiqn into two new condyles, one of which, covered by the squamosal on the outside, 

 articulates with the tegmen tympani ; whilst the other, retreating very sensibly, 

 coalesces with the ear-sac further backwards and downwards, close in front of the exit 

 of the portio dura nerve. In the figure these parts are continuous; but the continuity 

 is kept up at present by new cells, and these younger cells are all soft indifferent tissue 

 as yet; their morphological differentiation will be explained in the next stage*. 



Behindthe stylohyal and some distance outside the promontory (pr.), the portio 

 dura nerve (7") is seen in section, an excellent land-mark for the stylohyal ; further 

 backward the compound 8th nerve (8", 8*) is seen in the " foramen lacerum posterius" 

 (f.l.p.) ; the hypoglossal (9) is enclosed in the upgrowing exoccipital cartilage (Plate 

 XXX. figs. 3, 9, e.o.). 



The relation of the auditory sac to the exoccipital (e.o) is shown in fig. 7 ; the whole 

 arch of the horizontal canal is seen shining through the cartilage, and its ampulla is 

 obscured by the fibres of the portio mollis nerve (7*); the Gasseran ganglion, and the 

 compound 8th nerve are also severed (5, 8", 8*). 



Third Stage. — Embryo Pig, 1^ inch in length. 



Those metamorphic processes which were rapidly proceeding in the last stage have 

 become very complete in this, where the embryo is one third longer : this stage must be 

 copiously illustrated and described at length, as it is the best stepping-stone between the 

 early simple and the later transformed conditions.. The sections now to be described 

 are a series from the end of the snout to the occipital region. Parallel to each other, 

 yet they do not keep the same vertical relation to the embryonic head, but become 

 almost horizontal sections of the occipital region ; the whole head at this stage is about 

 equal in size to a horse-bean. 



The first of these slices is through the end of the snout (Plate XXXI. fig. 1), and 

 shows the coalescence of the alinasal cartilages with the backwardly bent trabecular 

 cornua [al.n., c.tr.). The next (fig. 2) is through the foremost part of the septum nasi 

 [s.n.) and valvular fold of the nostril — rudiment of alinasal turbinal [al. tb.) : a more 

 enlarged view of the lower half of the septum (fig. 2") shows the large and massive 

 trabecular cornua, and the prenasal part of the trabecular commissure between them. 

 In the next (figs. 3 & 3") the cornua 'are now seen to be retral, for here they are 

 becoming separate from the " prenasal ;" still the base of the septum nasi as well as their 



* I have studied the development of this interliyal tract in the Batrachia Anura and Ophidia, where it 

 never even chondrifies ; in the Eel (Anguilla), where it is very small and indistinct as cartilage, and fades into 

 a mere ligament ; in the Osseous Fish (Salmo solar) and the Ganoid (Accipenser sturio), where it becomes an 

 ossified rod of cartilage ; and in the embryo of Linota cannabina, where it chondrifies after a time and fuses 

 together again the incus and stylo-hyal. 



