216 PEOF. W. K. PAEKER ON THE 



a small epi-liyal soou appears, and soon loses its independence ; for it unites with the 

 descending ray of the columella, the infra-stapedial. In my figure of the columella of 

 the old Eowl (oj). cit. plate 87. fig. 3) the descending ray is a uniform narrow hand ; in 

 Professor Huxley's figure (Proc. Zool. Soc. 1869, p. 399, fig. 5) it is dilated helow; that 

 dilated part was a separate epi-hyal; much of the cerato-hyal {c.hy.) is aborted, and 

 the lower, or hypo-hyal part of it, is enclosed in the sagittate tongue. 



III. — The Vertebral Chaiu of the Chick, Stage 1. (1 week's incubation.) 



Por an account of the development of the vertebral chain up to this stage, the reader 

 is referred to Poster and Balfour's ' Embryology ' (1874). 



The formation of hyaline cartilage is my " cue " in these researches ; I enter into the 

 labour of the Embryologist when the various tissues that form the creature are fairly 

 difl^erentiated ; and then creep cautiously, along skeletal lines, leaving the other parts to 

 other workers : if we are to conquer this territory, we must divide it. 



The basal cartilage of the hind skull, parachordal, or " investing mass " (Plate XXIII. 

 fig. 1, iv.), although not segmented, is homologous with the vertebral centra, and even in 

 it the notochord («.c.) shows a disposition to become monUiform ("Powl's Skull," 

 plate 82. fig. 3, nc.). The greatest amount of modification of the vertebral chain occurs 

 directly beliind the occipital arch, and in that part itself the paired parachordal 

 cartilages unite round the notochord to form the single occipital condyle, which is 

 bilobate in most of the Prsecoces, but is a neat hemisphere in most of the Altrices. 



There is, in the Urodeles, a remarkable foreshadowing, so to speak, of the j)ivot- 

 joint, which is formed in addition to the proper occipito-atlantal articulation. In them 

 (see Trans. Linn. Soc. ser. 2, vol. ii. plates 20, 21, ocl.o.) the so-called odontoid process 

 of the axis vertebra is represented by an abortively-developed vertebra, which is formed 

 between the occipital condyles and the practical " atlas," namely the vertebra which 

 carries the skull. When we reach the Amniota, the first vertebra has its lower part 

 and its arch divided from the epiosteal centrum, or that part which is formed round the 

 notochord. That " core," set free, unites with the second vertebra — the axis or pivot- 

 vertebra ; and the so-called " centrum " of the atlas, or first vertebra, is a mere " inter- 

 centrum." This part gets its own osseous centre, and behind it, in the fore part of the 

 axis, another intercentrum is formed. After that the core of the atlas becomes 

 ankylosed to that of the axis ; and thus the pivot-joint is added to the very mobile 

 procoelous articulation of the imperfect atlas with the occipital arch. In the Powl these 

 two highly modified vertebrae (Plate XXIII. fig. 1, at., ax.), the atlas and axis, are devoid of 

 the small riblets {c.r.) which are developed from the 3rd onwards. In the Duck tribe 

 (Anatidse) and in most of the Hallidte they are present even in the 1st and 2nd cervical 

 vertebra? (see Proc. Roy. Soc. 1888, pp. 478, 479, and Proc. Zool. Soc. 1889, p. 173). 

 At this stage (Plate XXIII. figs. 1-3) the neural arches {n.a.) are incomplete above; they 

 are direct continuations of the cartilage that enrings the notochord, and do not, in the 

 cartilaginous condition, show their primary independence. In the lower Vertebrata 



