640 



THE PBOGTOD.EUM. 



shewn in the diagram (fig. 428). From the epithelial ridge three enamel 

 organs are represented as being developed. Such an arrangement may 

 occur when teeth are successively replaced. The lowest and youngest 

 enamel organ (e) has assumed a cap-like form enveloping a dental papilla, 

 but 110 calcification has yet taken place. 



In the next stage a cap of dentine has become formed, while in the 

 still older tooth this has become covered by a layer of enamel. As may be 

 gathered from this diagram, the primitive epithelial ridge from which the 

 enamel" organ is formed is not necessarily absorbed on the formation of a 

 tooth, but is capable of giving rise to fresh enamel organs. When the 

 enamel organ has reached a certain stage of development, its connection 

 with the epithelial ridge is ruptured (fig. 428). 



The arrangement represented in fig. 428, in which successive enamel 

 organs are formed from tho same epithelial ridge, is found in most Verte- 

 brata except the Teleostei. In the Teleostei, however (Tomes), a fresh 

 enamel organ grows inwards from the epithelium for each successively 

 formed tooth. 



The Proctod(JBum. 



In all Vertebrata the cloacal section of the alimentary tract 

 which receives the urinogenital ducts is placed in communication 

 with the exterior by means of an epiblastic imagination, constituting 

 a proctodseum. 



This invagination is not usually very deep, and in most instances 

 the boundary wall between it and the hypoblastic cloaca is not 

 perforated till considerably after the perforation of the stomodaeura ; 

 in Petromyzou, however, its perforation is effected before the mouth 

 and pharynx are placed in communication. 



The mode of formation of the ptoctodseum, which is in general 

 extremely simple, is illustrated by fig. 420 an. 



In most forms the original boundary between the epiblast of 

 the proctodivum and the hypoblast of the primitive cloaca becomes 

 obliterated after the two have become placed in free communication. 



7H C 



FIG. 429. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR 

 OF AN EMBRYO BlRD AT THE TIME OF THK FORMATION OF THE ALLANTOIS. 



cp. epiblast; Sp.c. spinal canal; ch. notochord; n.c. neurenteric canal; In/, hypo- 

 blast; p.a./i. postanal gut ; pi: remains of piiuiitive streak folded in on the ventral 

 side; al. alluntois; me. mesoblast; an. point where anus will be formed ; p.<: peri- 

 visceral cavity; inn. amnion ; so. somatopleure ; *]>. splanclmopleure. 



