MOEPHOLOGT OF TUE GALLIXACE.E. 215 



II. — Stage 1. Skull and Visceral Arches of Chick after one week's incubation *. 



The skul], at this stage, corresponds with the 2nd stage in my early paper. As it has 

 abeady been described there, I will merely remark here that the prochordal tract is 

 made up of three " trabccuIiB," the posterior paii-ed bars and the anterior azygous rod or 

 " interti'abecula," which ends in front as the prenasal rostrum, the part on which 

 the large premaxillaries are modelled. The paired trabeculre end in front as small 

 alse, called in my former paper " super-vomerine alae." The lower face, sho^^Ti in this 

 fig\u"e (Plate XXII.), has no true cartilage in the palatine region, although that tissue is 

 developed, to some degree, in Passerine and some other Birds. There is, therefore, in the 

 chondrocranium of the Chick no cartilaginous palato-quadrate, only a quadratum, 

 whose pedicle is free above and the " otic process " of which, in the Chick, is not 

 bifm-cated above, but simply gains a small articular facet on its inner side, for articula- 

 tion with the periotic capsule : this is true of the " Alectoropodes " — Phasianine and 

 Tetraonine Powls ; but in the " Peristeropodes " — Curassows and Mound Makers 

 (Cracidse and Megapodidse) — the head of the quadrate is double. 



In Birds, as in Pieptiles and many of the Ichthyopsida, the quadrate is formed, from 

 the first, as a separate cartilage from the free mandible, or articulo-Meckelian rod. 

 This is not always the case in the Salmon (Phil. Trans. 1873, plate 2, p. 123) ; and in 

 Marsupials (e. g. Macrojms major) the whole of this 1st visceral or intra-branchial arch 

 is developed as a single rod of cartilage, which becomes segmented, afterwards, into an 

 epi- and a cerato-branchial element. In the Marsupials and in all the Mammals the 

 1st visceral arch, as well as the 2nd, or hyoid, becomes arrested and largely devoted to 

 auditory purposes ; thus the quadrate becomes the incus, and the articular end of the 

 primary mandible becomes the malleus f . 



In my former paper I figtired the columella ( = stapes and hyomandibular) as the 

 only part found (proximally) in the 2nd or hyoid arch. That is true of this stage ; but 



* The Stages of Growth in the Limbs of tJie Fou'J. — ^To be sure of my steps in this investigation, I have followed 

 the growth of the limbs through a long series of stages : these are arhitranj ; the 1st answers to the older chicks of 

 my 2nd stage in the memoir on the Fowl's SkuU (Phil. Trans. 1S69, plate 81, pp. 761-76S). 



a. Stage 1. Embryo of 7 days' incubation. 



h. „ 2. ,, 8 „ 



c. „ y. „ 10 



;^ ., 4. „ 12 



e. „ o. ,, 14 or 15 „ 



/. „ 6. Chicken 2 or 3 days old. 



g. „ i. „ o3 „ 



A. „ 8. „ I year old. 



i. „ 9. Old Fowl. 

 t Some biologists, through want of embryological knowledge, are in doubt as to how the primordial mammal 

 could make nse of its mouth whilst a new hinge was being made to its jaws. My answer takes the form of a ques- 

 tion,f namely, What does the Tadpole do when its terminal, suctorial mouth is being changed into that of the widely 

 gaping aperture it possesses after metamorphosis ? 



33* 



