DEVELOPMENT OE THE SKULL OF THE COMMON FOWL. 
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coral-like in appearance, and a row of feeble osselets lying along the grooved mid line 
are ready, in a few hours , to connect together the whole basitemporal tract. Not only 
do these symmetrical plates rapidly coalesce, but the “ rostrum ” also will have united 
with them by the time they have melted into each other; then the “ parasphenoid ” 
of the Ichthyopsidan will be fully represented, and the open pituitary space will be 
filled in below by the extraneous plate. It cannot be void of interest that the ornithic 
embryo is thus seen to arrive at its further goal by passing, at first, along Ichthyopsidan 
paths ; not only is this seen in the skull-base, but also in all the outworks of the face. 
The anterior termination of the “ rostrum ” is very thin ; it lies in the midst of a 
large mass of stroma (Plate LXXXII. fig. 7, r.b.s.) underlying the perpendicular eth- 
moid (eth.), and above and between the laminar palatines (pa.) : this section shows well 
how thorough the amalgamation of the trabeculae becomes. Conjugation of symmetrical 
bands of simple cartilage at the mid line is a frequent phenomenon in the development 
of the skeleton. 
Where the moieties of the “ investing mass ” embrace the pituitary body, and thus, 
become trabeculae (all the prenotochordal part of the early skull-base is trabecular),, 
there the cartilage grows upwards into the alisphenoid (Plate LXXXII. fig. 4, a.s.), and 
downwards to form a steep wall to the pituitary space ( p.t.y .) : these walls are flanked 
by the “lingulae” ( l.g .) ; their nature, as outgrowths of the trabecular roots, and the 
dark appearance of their interior, is shown in this figure. 
Fourth Stage. — Head of Embryo 10 to 12 lines long: end of 2nd, and beginning of 
Zrd Week of Incubation. 
The primordial skull undergoes very rapid changes day by day ; this will be seen at 
once if the figures in Plate LXXXII. are compared with those in Plate LXXXIII. 
AH the figures in Plate LXXXIII. and figs. 8-12 in Plate LXXXII. are from a chick 
at the commencement of the third week of incubation, the head being an inch in length, 
or one-third larger than that from which the figures 1 to 7 in Plate LXXXII. are given ; 
this stage, however, will be drawn nearer to the last by showing the structure of some 
younger embryos. 
The basal view of the skull cannot be shown as merely primordial ; so that fig. 1 in 
Plate LXXXIII. must be compared with figs. 1 & 2 in Plate LXXXII. : in this latter 
Plate the inner view of the skull-floor is only partially given, whilst in Plate LXXXIII. 
fig. 2 I am able to show the entire primordial skull from above, the grafted splints 
below not being focused. 
But one of the most important changes to be noticed, and which was commencing 
but not described in the third stage, makes it profitable to compare the second with the 
fourth. We saw that in the second stage (Plate LXXXI. figs. 4 & 5) the cranio-facial 
axis had attained to a Struthious condition ; in Plate LXXXIII. fig. 4 this part is seen to 
be rapidly passing into the Tinamine stage, the region common to the ethmoid and nasal 
septum having become largely fenestrate ; this is intermediate between what is seen in 
