578 EMBRYONIC MEMBRANES chap- 



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. 155) composed 

 of ectoderm with a core of mesoderm, and their skeleton 

 arises by the formation of cartilage at their bases, which 

 extends inwards to form the arches, and outwards to form 

 the skeleton of the free portions of the hmbs. 



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 (cartilage-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, AUantois and Placenta.^ 



We must now consider some important and characteristic 

 structures which are developed in the embryos of the higher 

 Craniata (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 

 takes no direct share in the formation of the embryo 

 (Fig. 154). The extension of the ectoderm and endoderm 

 takes places regularly and symmetrically,- but the meso- 

 derm, while extending equally in the lateral and posterior 



