596 EMBRYONIC MEMBRANES CHAP. 



eventually meeting to a greater or less extent above the 

 brain : there is never, however, a complete cartilaginous roof 

 to the cranium, parts of which are only membranous and 

 form the fontanelles (pp. 43 and 436). In the meantime 

 the cartilaginous sense-capsules are developed, the olfactory 

 and auditory capsules uniting with the brain-case in front 

 and behind respectively. 



The visceral skeleton is formed as a series of cartilaginous 

 bars within the visceral arches, the first of which forms the 

 mandibular arch, the second the hyoid, and the others the 

 branchial arches. 



The limbs appear as small buds (Fig. 166) composed 

 of ectoderm with a core of mesoderm, in which their 

 skeleton arises by the formation of cartilage extending 

 inwards to form the arches, and outwards to form the 

 skeleton of the free portions of the limbs. 



As we have seen, the endoskeleton may remain more or 

 less entirely cartilaginous in the adult (e.g. Dogfish), but in 

 higher forms extensive processes of ossification set in, certain 

 bones replacing this cartilage to a greater or less extent, and 

 others being formed in the surrounding connective-tissue 

 (compare p. 43). 



Development of the Amnion, Allantois and Placenta. 



We must now consider some important and characteristic 

 structures which are developed in the embryos of Reptiles, 

 Birds, and Mammals, and known as embryonic membranes. 

 Taking the chick as a convenient example, these are formed 

 as follows. 



The blastoderm gradually extends peripherally so as to 

 cover the yolk, and thereby becomes divisible into an 

 embryonic portion, from which the embryo is formed, and 

 an extra-embryonic portion which invests the yolk-sac and 



