318 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



in the form of paired basal cartilages which are eventually continuous 

 throughout parachordal and trabecular regions but which may for a 

 time consist of separate portions lying one in front of the other; As 

 chondrification spreads from each of these primary elements, they 

 become united together in a continuous plate of cartilage, forming 

 the floor of the chondrocranium. From this in turn chondrification 

 spreads upwards to form the side walls and roof, and forwards into 

 the ethmoid and nasal regions. 



To the brain-case so formed there become added the protective 

 capsules of the olfactory organ and otocyst. As each of these organs 

 is a development of the external skin, we may assume with a con- 

 siderable degree of probability that their cartilaginous capsules were 

 originally independent of the cranium. 



Any repetition however of this completely independent stage of the 

 sense-capsules in question has apparently become obliterated from 

 ontogenetic development. Portions of the sense capsule may arise 

 from separate centres of chondrification e.g. in the case of the auditory 

 capsule the first rudiment may be in the form of an independent 

 patch of cartilage in the region of the lateral semicircular canal. 

 Even in such cases however the inner portion of the capsule develops 

 in continuity with the chondrocranium. Again the Dipnoan arrange- 

 ment, where the otic capsule is without any wall upon its mesial 

 side so that it takes the form merely of a bulging of the lateral 

 cranial wall, is to be looked upon as secondary. 



Skeleton of the Visceral Arches. — The anterior portion of 

 the alimentary canal forms a tube leading from the mouth back 

 underneath the cranium, its lateral walls perforated, and therefore 

 weakened, by the visceral clefts. The coelomic space being no longer 

 present in this region somatopleure and splanchnopleure are in 

 continuity, a continuous mass of mesenchyme extending from ectoderm 

 to endoderm. This mass of tissue is divided by the clefts into 

 the series of visceral arches and each of these is characteristically 

 strengthened by a tract of tissue in its interior undergoing con- ' 

 densation and chondrification to form half-hoop shaped cartilaginous 

 arches. These arches are named according to the mesenchymatous 

 arch in which they he — Mandibular (I), Hyoid (II) and First 

 branchial (III), Second branchial (IV) and so on. 



The skeletal branchial arches differ in number in different verte- 

 brates, just as do the corresponding mesenchymatous arches (see p. 

 153). In the Lamprey, 1 which probably in this respect shows the most 

 nearly primitive arrangement, the two half-hoops of a pair become con- 

 tinuous with one another ventrally. In the gill-breathing fishes the 

 hoop typically becomes divided by joints into four segments on each 

 side, with a median ventral copula — no doubt an adaptive arrange- 

 ment to facilitate the movements of respiration. Where branchial 

 respiration is reduced the arch has reverted to its primitive un- 



1 There is in the writer's opinion no sufficient evidence to doubt that the visceral 

 skeleton of Cyclostomes is homologous with that of Gnathostomes. 



