INTRODUCTION 457 



Amphioxus, in the frog, and in forms having rounded gastrulae, while in the 

 chick the neural area is precocious. Viewed in their totality, however, the 

 tubulations of all of the major organ-forming areas are simuhaneous processes 

 with the exception of the notochord which does not become tubulated but 

 continues as an elongated rod of cells. 



b. Increase in Size and Antero-posterior Extension of the Tubulated, 

 Major Organ-forming Areas 



Another goal to be achieved by the embryo during the immediate, post- 

 gastrular period is an increase in size, together with an antero-posterior exten- 

 sion of the major organ-forming areas. These changes are associated with 

 tubulation, and they aid in producing the elongated, cylindrical form typical 

 of the chordate body. 



c. Regional Modifications of the Tubulated Areas 



As tubulation of the various major organ-forming areas progresses, specific, 

 organ-forming areas or fields (see end of chapter), located along the respec- 

 tive primitive body tubes, begin to express themselves and develop in a spe- 

 cialized manner. Thus, regional differentiation of the major organ-forming 

 areas, comprising each primitive body tube, is another feature of the post- 

 gastrular period. As a result, localized areas along each of the body tubes 

 show changes in shape, and specific, individualized structures begin to make 

 their appearance. For example, the neural tubulation develops the primitive 

 parts of the brain at its anterior end, while the posterior portion of the neural 

 tube, caudal to the brain area, begins to form the spinal cord. Thus, the 

 primitive brain becomes a specific peculiarity of the head region. Also, the 

 epidermal tubulation at its cranial end contributes definite structures peculiar 

 to the head. In the pharyngeal region, special developmental features arise in 

 the entodermal tube together with the epidermal tube and the mesoderm. In 

 the trunk region, modifications of the entodermal and mesodermal tubes give 

 origin to many of the structural conditions peculiar to this area, while in the 

 tail, the neural and epidermal tubulations together with activities of the meso- 

 derm account for the characterstic structures of the tail appendage. These 

 special developmental features of the respective, tubulated, organ-forming 

 areas, which arise in specific areas along the antero-posterior axis of the 

 embryo, occur in much the same way throughout the vertebrate group with 

 the result that common or generalized structural conditions of the tubulated 

 organ-forming areas appear in all vertebrate embryos. That is, the primitive 

 brains of all vertebrate embryos up to a certain stage of development resemble 

 each other in a striking manner; the contributions of the epidermal tubulation 

 to the head also resemble each other, and the early development of the 

 pharyngeal and trunk regions is similar. As a result, the early morphogenesis 



