TUBULATION OF ORGAN-FORMING AREAS 471 



the formation of the neurenteric canal that two important changes occur in 

 the future hindgut area of the archenteron at this time, namely, the posterior 

 dorsal end of the archenteron projects dorso-caudally to unite with the neural 

 tube (fig. 220D), while the posterior ventral end of the archenteron moves 

 ventrad toward the epidermis where it meets the epidermal invagination, the 

 proctodaeum (fig. 220D). 



d. Anterior and Posterior Neuropores; Neurenteric Canal 



The fusion of the neural folds in the middorsal area proceeds anteriad and 

 posteriad from the anterior somitic and hindbrain region as described above. 

 At the anterior end of the forebrain when fusion is still incomplete, an opening 

 from the exterior to the inside of the neural canal is present; it forms the 

 anterior neuropore (figs. 229D; 23 IL; 235B; 242E-G; 245B). When fusion is 

 complete, this opening is obliterated. The caudal end of the neural tube closes 

 in a similar manner, and a posterior neuropore is formed (figs. 242E, G; 

 245). In the chick, as in the mammal, the posterior neuropore at first is a 

 wide, rhomboidal-shaped trough, known as the rhomboidal sinus. The an- 

 terior end of the primitive streak is included within the floor of this sinus 

 rhbmboidalis (fig. 235 A, B). The point of posterior neuroporal closure is at 

 the base of the future tail in most vertebrates (fig. 245B), but, in the elasmo- 

 branch fishes, this closure is effected after the tail rudiments have grown 

 caudally for some distance (fig. 229B-E). 



The vertebrate tail arises from a mass of tissue, known variously as the 

 tail bud, caudal bud, or end bud, and the posterior end of the neural tube 

 comes to lie in the end-bud tissues (figs. 225, 238C). The end bud grows 

 caudally and progressively gives origin to the tail. It consists of the following: 



(a) the epidermal tube (i.e., the ectodermal covering of the end bud); 

 within this epidermal layer are 



(b) the caudal end of the neural tube; 



(c) the caudal end of the notochord; 



(d) mesoderm in the form of a mass of rather compact mesenchyme sur- 

 rounding the growing caudal ends of the notochord and neural tube; 

 and 



(e) a caudal growth from the primitive intestine or gut. 



This extension of the gut tube into the tail is called, variously, the tail gut, 

 caudal gut or post-anal gut. It varies in length and extent of development in 

 embryos of different vertebrate species. In some species it is joined to the 

 neural tube; in others it is not so united. For example, the tail gut is as long 

 as the trunk portion of the gut in the young shark embryo of 8 to 10 mm. in 

 length, and at the caudal extremity it is confluent with the neural tube (figs. 

 21 7A; 229F). The confluent terminal portions of the neural and gut tubes 

 form the neurenteric canal. This well-developed neurenteric canal extends 



