6 Transactions of the 



III. — On the Development of the Skull in the Tit ancV Sparrow- 

 Hawk. By W. K. Parker, F.K.S. 



(Read before the Royal Microscopical Society, Dec: 4, 1872.) 



Part I. 



Plate II. 



In an embryo of one of our native Titmice (either Partis cerulseus, 

 P. fringillago, or P. ater — one of the three), I found much to 

 interest me : it had undergone about three-fourths of the incubating 

 process. The head (Plate II., Fig. 1), the size of which makes it 

 most probable that it belonged to P. fringillago — the Ox-eye Tit — 

 had fine filamentous feather-tips streaming from it here and there : 

 it had, already, the characteristically small neb, so that I should 

 have guessed, if I had not known for a certainty, that it was a Tit- 

 mouse embryo. It is difficult to say whether the Histology or the 

 Morphology of this little cranium is most interesting. Already the 

 cartilaginous skull was well formed (Fig. 2), but, like that of the 

 Lepidosiren, and some of the Tailed Amphibia, it had very little 

 bony matter belonging to the " endoskeleton " : the " investing " or 

 borrowed bony tracts were fairly on their way towards typical 

 completion. One true internal bone was present, namely, the 

 " basi-occipital " (Figs. 2 and 3, b. o.), a spear-head-shaped bony 

 tract, formed round the notochord (n. c.) in its sheath, and already 

 leavening the " investing mass," or basilar plate of cartilage (i. v.) 

 on each side : this mass is the undivided counterpart of the blocks 

 that unite at the mid-line to form the bodies of the vertebras. The 

 foramen for the hypoglossal nerve (9) and its near neighbour the 

 " posterior condyloid foramen," are clearly seen, and also the larger 

 holes farther forwards and outwards for the " vagus " (8). The 

 " notochord " (Figs. 2 and 3, n. c.) is well seen ; it scarcely reaches 

 the fore-end of the " basi-occipital ossicle " (b. o.), and is seen running 

 through the axis of the berry-shaped occipital condyle (o. c). On 

 each side, the occipital cartilage is of great size, and is flanked above 

 by the investing " squamosals " — " squaroae temporis "; below, the 



DESCRIPTION OF PLATE II. 



Fig. 1. — Head of Tit (Paras fringillago'?), natural size. 

 2. — Lower view of skull of ditto, X 7. 

 3. — Part of same, x 30. 

 4. — Palate of same, x 7. 

 5. — Mandible of ditto (inner view), X 7. 

 6. — „ „ (outer view), x 7. 



7. — Part of orbital septum, x 30. 

 8. — Fore-part of same object, x 150. 

 9. — Hind-part of ditto, X 150. 



