406 GASTRULATION 



of gastrulation, although mitoses occur in other regions as well. During 

 later stages of gastrulation, the entire complex of cells around the 

 blastoporal region divides actively. 



(c) Involution. Notochordal cells converge to the midregion of the dorsal 

 blastoporal lip and then turn inward (involute) over the lip area to 

 the inside. 



(d) Extension. General elongation of the embryonic rudiment as a whole 

 occurs, including the neural plate area. 



(e) Convergence. Mesodermal cells converge toward the middorsal area 

 of the blastopore. The path of this convergence is along the lateral 

 lips of the blastopore, particularly the inner aspects of the lips. This 

 movement is pronounced toward the end of gastrulation when each 

 half of the mesodermal crescent moves dorsad toward the middorsal 

 area of the blastopore. The mesoderm thus comes to lie on either 

 side of the notochordal material at the dorsal lip of the blastopore. 



(f) Constriction of the blastopore. During later phases of gastrulation, the 

 blastopore grows smaller (fig. 247A-D), associated with a constriction 

 of the marginal region of the blastoporal opening, particularly of the 

 entodermal and epidermal layers. The movement of the mesoderm de- 

 scribed in (e) above plays a part in- this blastoporal change. 



2) Epiboly. The caudal growth of the entire ectodermal layer of cells, epi- 

 dermal and neural, and their antero-posterior extension is a prominent feature 

 of gastrulation in Amphioxus. 



(Further changes in the late gastrula, together with the closing of the 

 blastopore, are described in Chapter 10. See tubulation of neural plate, etc.) 



2. Gastrulation in Amphibia with Particular Reference 



TO THE Frog 



a. Introduction 



1) Orientation. A line drawn from the middle region of the animal pole to 

 the midvegetal pole constitutes the median axis of the egg. In the anuran 

 Amphibia the embryonic axis corresponds approximately to the egg axis. That 

 is, the midanimal pole of the egg represents the future anterior or antero- 

 dorsal end of the embryo, while the midvegetal pole area denotes the posterior 

 region. 



As indicated previously (Chap. 7), the very late blastula is composed of 

 presumptive organ-forming areas arranged around the blastocoelic space. The 

 yolk-laden, future entodermal cells of the gut or digestive tube form the 

 hypoblast and are concentrated at the vegetal pole. Presumptive notochordal 

 and mesodermal cells constitute a marginal zone of cells which surrounds 

 the upper region of the presumptive entodermal organ-forming area (fig. 

 169C-F). The presumptive notochordal area is in the form of a crescent. 



