Royal Microscopical Society. 103 



outstanding point of structure — the development of its occiput — ■ 

 its members agree with the Lizard, and with reptiles generally, 

 and not with the other birds — even the other Passerines. 



I have shown the earlier condition of the Passerine skull in my 

 former papers — on the Crow and Titmouse, and similar studies of 

 the skull in embryos of the Song-Thrush (Turdus musicus) have 

 been made by me, but not figured. The earliest of these here 

 given is that of the Storm-cock {Turdus viscivorus), three or four 

 days before hatching (Plate VIII.). Here the skull is seen to be far 

 developed, although the inworks are mostly composed of cartilage, 

 and the oatworks of very thin membrane-bones, easily peeled off 

 the spirit-preparation. Looking at the basis-cranii from below 

 (Figs. 2 and 5), we see that the notochord has become ensheathed 

 by a bony deposit; the basi- occipital (b. o.) has the form of a 

 scale of wheat- chaff. This new bone is surrounded by cartilage 

 common to the " investing mass " and the auditory capsule, and in 

 the substance of the latter the rudimentary cochleae shine through. 

 On each side of the foramen magnum (/. m.) we have now a leafy, 

 lobate bone, enfolding the vagus (8) as it passes out between the 

 auditory and occipital cartilaginous regions. This lime-hardened 

 tract of the occipital wall passes upwards on each side the great 

 hole for the spinal chord, but the keystone piece has not appeared 

 (Fig. 3, s. o.). Undergirding the skull-base, hiding part of the 

 cochlear cavities, and forming an extraneous floor to the " pituitary 

 space," we see the basi-temporals (b. t.) like an inverted saddle ; 

 they now form one bone by median union of substance. This 

 bone is notched in front and behind — the remains of the interspace 

 between the two original pieces, and also, laterally, they are 

 fenestrate ; the double top in front underlies the converging Eusta- 

 chian tubes and the pituitary body. These bones, like the spiked 

 and winged bone in front of them — the parasphenoid proper — 

 are ossifications of a thick felt of connective fibrous tissue, which, 

 however, was calcified very early, and whilst the cells were scarcely 

 spindle-shaped. 



The parasphenoid (pa. s.) is a long scoop-shaped bone, ending 

 behind in wings that rival the basi-temporals ; these wings are 

 extravagant periosteal outgrowths from the bony matter ossifying 

 the apices of the trabecular bars. 



The whole bone is an azygous splint applied to the under-face 

 of the soon-coalesced moieties of the first visceral arch — the trabe- 

 cular arch ; and the wild growths behind are economized to form the 

 anterior recess of the complex tympanic cavity — the primary 

 condition of which cavity was the first post-oral visceral cleft — a 

 crack, as it were, or dehiscence in the facial wall of the embryo. 

 Strange as it may seem, the " parasphenoid " is a facial, and not 

 a cranial bone, and the harmonious co-working of these facial bars 



