606 AMNION CHAP. 



The limbs appear as small buds (Fig. 158, C) composed 

 of ectoderm with a core of mesoderm, in which latter 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 

 practically entirely cartilaginous in the adult (e.g. dog- 

 fish), but in higher forms extensive processes of ossifica- 

 tion set in, certain bones replacing the 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, as we have seen (p. 585 and Fig. 

 159), 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. The 

 extension of the ectoderm and endoderm takes place 

 regularly and symmetrically ; but the mesoderm, while 

 extending equally in the lateral and posterior regions, 

 grows forwards in the form of paired prolongations 

 which afterwards unite, so that for a time there is an 

 area of the blastoderm in front of the head of the 

 embryo formed of ectoderm and endoderm only and 

 called the pro-amnion (p. 583 and Fig. 153, pr. am). 



Before the embryo has begun to be folded off from 

 the yolk the rudiment of one of the two embryonic 

 membranes, the amnion, has appeared. A crescentic 



