a:-ygou3 cartilaginous bud at the mid-line of the fronto- nasal process ; it is about to become the axis of the beak, and will turn forwards and upwards. This is the prenasal or basi- trabecular cartilage, a notable morphological element up here, amongst the Birds, and down amongst the lower or cartilaginous types of Fishes. At the present it corre sponds exactly to the state of the Green Turtle s skull (chondro- cranium) at the time of hatching, and in that type it stops at this stage, not raising itself into a forth-
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FIG. 4. Skull of Chic* whose head was five lines
long more than half of bird s-eye view, x 12 cliamcters- Lettering as above, with addition of i.v., investing mass, or parachordal cartil- a e > * ( internal carotid artery; 8, foramen neaa, ab in d, OKdLt, L. f or vagus nerve. Here the lingula (Ig.) shows
the axis the a P ex of the trabecula externally.The mouth, which in the first stage was a large four- sided cleft, with nrorhioed corners W1U1 prC QUCea corners, and clean beneath the ? now a space, of which points more forward than downward, a good step towards its eventual direction. The further development of the facial arches will be best studied in older specimens at the end of this, the second stage ; but at its commence ment there is wanted the condition of the antero-superior segment of the hyoid, or second post-oral arch. This has not been worked out in the Fowl s chick, but in an embryo of the House Martin (Chelidon urbica), corresponding to the stage given in our third figure as to development.
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Flo. 5. Facial arches of House Martin (Chelidon urbica), middle of incubation,
X 20 diameters, q., quadrate, with orbital process or "pedicle" in front, "otic process" behind, and articular facet below; ar., articular part of mandible; mle., Meckel s cartilage (cut through); m.st., medio-stapedial (the line from the letters only reaches half-way to this bar) ; i.st., supra-stapedial ; t.st., extra-stapedial; i.st., infra-stapedial (soft); stJi., stylo-hyal; t.e.o^ tym panic wing of exoccipital; tg., tongue; c.hy. cerato-hyal ; b.hy., basi-hyal; b.br., basi-branchial ; Ir. 1, 1st branchial or thyro-hyal.
In this instance the quadrate and articular Meckelian cartilages have acquired very nearly their proper form (fig. 5, q., ar., mk.), the suspensorial part having a free forward and inward looking process, the "pedicle," and a backward turned process, the "otic process," articu lating with the auditory sac (this is drawn as cut away in part, and only that which is conjoined to the occi pital cartilage (t.e.o.) is given). Just below and^ behind the otic process of the quadrate, exactly where in riper embryos we find the fenestra avails, having fitted into it the elegant oval base of the " columclla auris," there is in this instance a curious trowel of cartilage, continuous by the upturned end of its handle with the very substance of the ear-capsule. The solid sickle-shaped cartilage be hind this is the "tympanic wing" of the exoccipital (t.e.o.) Here we have the wanting upper and anterior segment of the hyoid arch, corresponding to the separately developed pterygo-palatine bar, its " serial homologue." The blade or free end of the trowel is concave on the under side, and is thick above and at its margin on the outside ; it is pointed above and below. A ligament connects the upper point with the ear-capsule behind the quadrate, and an other tract of soft indifferent tissue reaches downwards to a small bar of cartilage, which looks backwards and a little downwards. This little cartilage (st.h.) is only connected with the distal piece (c.hy.) by fibrous tissue ; it is the stylo- hyal, and corresponds to what is permanent in the Crocodile. The bar itself is the " medio-stapcdial " (m.st.); it will soon segment itself off from the ear-capsule, bringing away with it an oval piece of the periotic wall ; that oval part is the true stapes. The broad part of the " blade " is the extra- stapedial, and on it will be stretched the membrani tym- 2)ani. It precisely corresponds now with that of that old Lacertian Hatteria or Sphenodon.[1]
The " stylo-hyal " will soon be conjoined to the extra- stapedial plate, and the (afterwards) chondrified band will be the " iufra-stapedial."[2]
In a few- hours the changes that have taken place in the chick s skull are very noticeable and important ; these are recorded as the end of the second stage.
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FIG. 6. Skull of Chick, second stage, head 5 lines long, X 6 diameters. Outer
view, with brain and roof removed to show chondro-cranium. Letters as in figs. 1-4, with the addition of al.n., alinasal cartilage; al.s., aliseptal ; al.e., aliethmoidal; p.p., pars plana; a. s., alisphunoid; p.f., post-f rental (sphenotic region); s.o., super-occipital; St., stapes; /.?., foramen rotundum; p.s.c., pos terior semicircular canal.
fast the life-processes are moulding the embryonic head into the fashion seen in the adult ; no trace of bone has appeared, save in the sides of the mandible. The roof of the skull never chondrifies, but is covered in by secondary bones. Like the skull of an adult Shark or Skate, the whole chondro-cranium is one continuous structure all save the post-oral arches. The auditory mass is now environed by cartilage stretching over the back of the head the occipi tal, and by wing-like growths that wall-in the bulk of the
brain behind the eyes the alisphenoid (a.s.) The high- ↑ See Huxley " On the Representatives of the Malleus and Incus of the Mammalia in the other Vertebrata," Proc. Zool. Soc., May 27, 1869, pp. 394-397, figs. 1-4.
- ↑ For a comparison of these parts with the ossicula auditus of the Mammal, see also Parker " On the Structure and Development of the Skull in the Pig," Phil Trans. 1874 i. 331.
