634 POSTANAL SECTION OF THE MESENTERON. 



there grow out numerous hollow diverticula into the passive splanch- 

 nic mesoblast. 



As the ductules grow longer and become branched, vascular pro- 

 cesses grow in between them, and the whole forms a compact glan- 

 dular body in the mesentery on the dorsal side of the alimentary 

 tract. The funnel-shaped receptacle loses its original form, and elon- 

 gating, assumes the character of a duct. 



From the above mode of development it is clear that the glan- 

 dular cells of the pancreas are derived from the hypoblast. 



Into the origin of the varying arrangements of the pancreatic 

 ducts it is not possible to enter in detail. In some cases, e.g. the 

 Rabbit (Kolliker), the two lobes and ducts arise from a division of 

 the primitive gland and duct. In other cases, e.g. the Bird, a 

 second diverticulum springs from the alimentary tract. In a large 

 number of instances the primitive condition with a single duct is 

 retained. 



Postanal section of the mesenteron. In the embryos of all the 

 Chordata there is a section of the mesenteron placed behind the 

 anus. This section invariably atrophies at a comparatively early 

 period of embryonic life ; but it is much better developed in the 

 lower forms than in the higher. At its posterior extremity it is pri- 

 mitively continuous with the neural tube (fig. 420), as was first shewn 

 by Kowalevsky. 



The canal connecting the neural and alimentary canals has 

 already been described as the neurenteric canal, and represents the 

 remains of the blastopore. 



In the Tunicata the section of the mesenteron, which in all probability 



corresponds to the postanal gut of the 

 71 -f Vertebrata, is that immediately following 



the dilated portion which gives rise to the 

 branchial cavity and permanent intestine. 

 It has already been shewn that from the 

 dorsal and lateral portions of this section 

 of the primitive alimentary tract the noto- 

 chord and muscles of the Ascidian tadpole 

 are derived. The remaining part of its 



FIG. 4-23. TRANSVERSE OPTICAL walls forms a solid cord of cells (fig. 423, 

 SECTION OF THE TAIL OF AN EM- '), which either atrophies, or, according 

 HRYo OF PHALLXJSIA MAMMILLATA. to Kowalevsky, gives rise to blood-vessels. 

 (After Kowalevsky.) In A mphioxis the postanal gut, though 



The section is from an embryo c i istinct i y developed, is not very long, and 

 of the same age as fig. 8 iv. , . J . i i 



, atrophies at a comparatively early period. 



cJi. notocnord ; n.c. neural T ., ,..,,.' ,. J . 



canal; me. mesoblast; /'. hypo- In Elasraobranctm this section of the 



blast of tail. alimentary tract is very well developed, 



and persists for a considerable period of 



embryonic life. The following is a history of its development in the genns 

 Scyllium. 



Shortly after the stage when the anus has become marked out by the 

 alimentary tract sending down a papilliform prccess towards the skin, the 



