640 



THE PROGTOD.EUM. 



shewn in the diagram (fig. 428). From the ejnthelial ridge tliree enamel 

 organs are represented as being developed. Such an arrangement may 

 occur when teeth are successively replaced. The lowest and yovingest 

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

 but no 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 the 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 sxxccessively 

 formed tooth. 



The Proctodeum. 



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 invagination, constituting 

 a proctodfeum. 



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 stomodfeum ; 

 in Petromyzon, however, its perforation is effected before the month 

 and pharynx are placed in communication. 



The mode of formaticm of the proctodfeum, which is in general 

 extremely simple, is illustrated by fig. 420 an. 



In most forms the original boundary between the epiblast of 

 the proctoda^um and the hypoblast of the primitive cloaca becomes 

 obliterated after the two have become placed in free communication. 



Fig. 429. Diageammatic longitudinal skction through thk posterior end 



OF AN EMBRYO BiRD AT THE TIME OF THE FORMATION OF THE AlLANTOIS. 



cp. epiblast; Sjj.c. spinal canal; ch. notochord; n.e. neurenteric canal; hij. liypo- 

 blast; p.a.fi. postanal gut ; p?'. remains of piimitive streak folde'l in on the ventral 

 side; al. allantois; me. mesoblast; an. point where anus will be formed ; p.c. peri- 

 viseoral cavity; am. amnion; no. somatopleure ; sp. splauchnopleure. 



