221 
Royal Microscopical Society. 
This more advanced stage has been obtained in an embryo of the 
Rook (C. frugilegus), two days older, or about the eighth or ninth 
day of incubation. The basal view of the skull (Plate XXXV., 
Fig. 1) shows several things not present, and others not figured in the 
earlier stage. The cartilaginous skull is well formed, and its first 
bony instalment is gained, namely, the “ basi-occipital” ( b . o.). This 
is formed as a delicate bony investment to the cranial part of the 
notochord, and then spreads laterally into the halves of the “ investing 
mass ” (i. v.). The whole occipital cartilage is now well seen with its 
condyloid foramen (9), and the passages for the “ vagus ” nerve (8). 
The condyle (o. c.) is more typically ornithic, being nearly hemispheri- 
cal ; the great tympanic alae also grow widely from the ex-occipital 
region ( e . o.), which is not ossified as yet. The super-occipital 
cartilage (s. o .) is a thin commissure uniting the huge ear sacs (au). 
This is still better seen in Fig. 2, where the anterior, horizontal, 
and posterior semicircular canals have stretched the auditory 
cartilaginous coat almost to bursting. The “ ali-sphenoidal ” 
cartilages grow up directly from the pituitary part of the “ investing 
mass” (Fig. 1 , py. iv.), but the space itself, embraced in front by 
the top ends of the trabeculae, is membranous below. So much of 
the “ basilar plate,” or investing mass, as is not ossified by the noto- 
chordal bony sheath still forms a broad floor with a free front edge. 
The “ posterior sphenoid ” here borrows — afterwards, from the 
symmetrical ossifications formed in the thick sub-mucous cushion 
which is stuffed between the fauces and basis-cranii. 
These bones, my “ basi-temporals,” are beautifully shown in 
this stage (Fig. 1, b. t.), as also the grooved bony style which 
yields hardening material to the fore-part of the basi-sphenoid ; 
this is the “ parasphenoid ” (pa. s.). Further forwards, the cartila- 
ginous base is seen between the palatines (pa.), and this becomes 
cleft, vertically, to form the hinge between the face and skull. 
The rest of the cartilage is the base of the nasal septum (s. n.), 
and the “ prae-nasal rostrum ” (p. n.). Besides the parasphenoid, the 
trabecular arch has the prae-maxillaries ; these are very much 
increased in development since the last stage (Figs. 3, 4, 5, p. x.). 
Seen from above (Fig. 4), they already meet, nearly cover 
the “ prae-nasal,” and send their nasal processes to the ethmoid, 
above. 
The vomerine splints were not well made out in this pre- 
paration, although I obscurely saw the division into two halves ; 
but in a Brown Linnet at this stage, I found them to be formed as 
“ endostoses,” in an oval patch of cartilage. Above the trabecular 
region (Fig. 3) the “ ali-nasals,” “ ecto-ethmoid,” and “ meso- 
ethmoids,” are shown as increasing in size and becoming perfect in 
form, and between the ethmo-presphenoidal plate (eth. p. s .) and 
the trabecular crest a thin longitudinal cleft has been formed, the 
