758 GENERATION AND DEVELOPMENT. 



placenta, contains the remnants of other structures which 

 in the early stages of the development of the embryo were, 

 as already related, of great comparative importance. Thus, 

 in early foetal life, it is composed of the following parts : 

 (l). Externally, a layer of the amnion, reflected over it 

 from the umbilicus. (2). The umbilical vesicle with its 

 duct and appertaining omphalo-mesenteric blood-vessels. 

 (3). The remains of the allantois, and continuous with it 

 the urachus. (4). The umbilical vessels, which, as just 

 remarked, ultimately form the greater part of the cord. 



DEVELOPMENT OF ORGANS. 



It remains now to consider in succession the develop- 

 ment of the several organs and systems of organs in the 

 further progress of the embryo. 



Development of the Vertebral Column and Cranium. 



The primitive part of the vertebral column in all the 

 Vertebrata is the gelatinous chorda dorsalis, which con- 

 sists entirely of cells. This cord tapers to a point at 

 the cranial and caudal extremities of the animal. In the 

 progress of its development, it is found to become enclosed 

 in a membranous sheath, which at length acquires a fibrous 

 structure, composed of transverse annular fibres. The 

 chorda dorsalis is to be regarded as the azygos axis of 

 the spinal column, and, in particular, of the future bodies 

 of the vertebrse, although it never itself passes into the 

 cartilaginous or osseous state, but remains enclosed as in a 

 case within the persistent parts of the vertebral column 

 which are developed around it. It is permanent, however, 

 only in a few animals : in the majority it disappears at an 

 early period. 



The cartilaginous or osseous vertebrse are always first 

 developed in pairs of lateral elements at the sides of the 



