v CHONDKOCKANIUM 315 



this is probably the most archaic portion of the cranial roof of the 

 Vertebrate. At the same time the possibility must not be lost 

 sight of that instead of being of ancestral significance this feature 

 may be associated merely with particular activity of cartilage- 

 formation in the region of the otocyst, connected with the need of 

 protecting that superficially placed organ of sense. 



Development of the Chondkoceanium in Birds. — According to 

 Sonies (1907) the first cartilage to make its appearance is an unpaired 

 plate arranged in a frontal plane and surrounding the notochord 

 at its highest point in the cerebral or mesencephalic flexure. Sonies 

 terms this (Fig. 157, acr) the acrochordal cartilage and states that 

 it makes its appearance, in the 5-day embryo of the chick. What 

 appears to correspond to it in Apteryx is described by T. J. Parker 

 as the prochordal cartilage, though in this case it lies quite anterior 

 to the notochord. Very soon after the acrochordal cartilage, the 

 parachordal makes its appearance — ensheathing the notochord. As 

 this is thickest laterally and very thin ventrally and especially 

 dorsally (where indeed it may be absent) it presents when viewed as 

 a transparency from the dorsal or ventral side a misleading paired 

 appearance. In Apteryx however the parachordals have apparently 

 retained the actual paired condition. 



For a time the parachordal and acrochordal cartilages are 

 separated by a wide gap but later (11-12 mm.) this becomes filled in 

 by the development of the paired elongated mesotic (basiotic) 

 cartilages (Fig. 157, B, mo). In the Duck these are at first 

 independent, but in the Chick they appear to be, even at the time of 

 their first appearance as cartilage, continuous with the parachordals. 

 Extending forwards they become continuous with the acrochordal, 

 bounding upon their mesial side a space in which no cartilage is 

 present — the posterior basicranial fontanelle (Fig. 157, E, p.b.f). 

 Postero-externally the mesotic cartilage fits round the lagena, form- 

 ing the rudiment of the cochlear part of the auditory capsule. 



The parachordal cartilage spreads out on each side forming the 

 basilar plate of cartilage and in embryos of about 7 days (13-14 mm.) 

 two pairs of neural arch-elements make their appearance as lateral 

 projections near its posterior end (Fig. 157, E, n.a) — the posterior, 

 situated between the Hypoglossal and the First Cervical nerve, 

 developing first. In the Kestrel (Tinnunculus alaudarius) Suschkin 

 (1899) found three such occipital arches (Fig. 158, n.a) and Gaupp 

 looks upon this as probably the typical number for Birds. 



The acrochordal spreads out and forms a transversely situated 

 plate of cartilage. 



The trabeculae appear in the chick embryo of about 11 mm. as 

 paired parallel rods of cartilage underlying the fore-brain. Posteriorly 

 each passes into a swelling lying lateral to the pituitary body and 

 as in the Duck and Starling (Sturnus) this forms at first an inde- 

 pendent piece Sonies terms it the polar cartilage. Even in the 

 Duck embryo this polar cartilage (Fig. 157, C, p) becomes very soon 



