474 



DEVELOPMENT OF PRIMITIVE BODY FORM 



EN0-8UD TISSUE 

 NEURENTEHIC 



OTOCHOHD 

 BNOTOCHORDAl 



LIVER OIVERTICULU 



Fig. 228. Sagittal section, showing organ-forming areas of Bombinator embryo. (After 

 O. Hertwig: Lehrbuch der Entwicklungsgeschichte des Menschen iind der Wirbeltiere. 

 1890. Jena, G. Fischer.) Observe elongated tail gut. 



vesicle, is a solid mass of cells. Thus, the shark and Bombinator embryos, on 

 the one hand, and the frog, chick, or mammal embryo, on the other, represent 

 two extremes in the development of the tail gut in the vertebrate group. 



In the reptiles, also in some birds, such as the duck, in the human embryo, 

 and certain other mammals, a transient notochordal-neural canal is present 

 which connects the enteron or gut tube with the caudal area of the forming 

 neural tube (figs. 200B, E; 207B; 231G-K). This canal is occasionally referred 

 to as a neurenteric canal. However, it is best to view this condition as a special 

 type of development within the above group, for it is not strictly comparable 

 to the neurenteric canal formed in the developing tail of the embryos of the 

 frog, shark, etc., where the neurenteric canal is formed by a definite union 

 between neural and tail-gut tubes as they project caudalward into the tail 

 rudiment. 



2. Epidermal Tubulation 



The formation of the external, epidermal, tubular layer of the vertebrate 

 body is a complex procedure. Its development differs considerably in the 

 rounded type of gastrula of the Amphibia from that in the flattened gastrula 

 of the chick or mammal. 



a. Development of the Epidermal Tube in Amphibia 



In the frog and other Amphibia, tubulation of the epidermal area of the 

 blastula begins during gastrulation. At the end of gastrulation, the changes 

 involved in epiboly have transformed the ectodermal area of the blastula into 

 an oval-shaped structure, surrounding the internally placed mesoderm and 

 entoderm (fig. 219). The neural plate material occupies the middorsal area 

 of this oval-shaped, ectodermal layer, while the future epidermal area forms 

 the remainder. Following gastrulation, the anterior end of this oval-shaped 

 structure, in harmony with the forming neural tube, begins to elongate and 



