The Development of the Vertebrate Embryo 57 



The early cleavages are regular phenomena that occur at specified 

 times after fertilization. Thus in the egg of the frog, Rana sylvatica, in- 

 cubated at 18°C, the first cleavage (longitudinal) occurs at 2.5 hours 

 after fertilization, the second (also longitudinal) occurs at 3 hours, the 

 third (transverse) at 4.5 hours, the fourth (again longitudinal) at 5.5 

 hours, leaving the embryo as a still solid ball consisting of two hemispheres 

 of eight cells each, the dorsal being smaller than the ventral. The blastular 

 cavity soon appears and produces a hollow ball with a thin roof and a 

 thick floor. By 21 hours the blastula is complete and gastrulation com- 



Gastrulation 



At the beginning of gastrulation, the embryo is a hollow ball of 

 cells. By the end of gastrulation, the embryo consists of three basic tissue 

 layers : an outer layer ( ectoderm ) , an inner layer ( endoderm ) , and an in- 

 termediate layer (mesoderm). Gastrulation, then, involves a series of 

 integrated cell movements that lead to the formation of these discrete 

 layers. Because the movements are quite complex, they are hard to follow. 

 Investigators have observed them by time-lapse movie photography and 

 by marking areas of the blastula with carbon particles or dyes and follow- 

 ing the path of motion. In this way they have been able to map the origins 

 of the three tissue layers back to cells of the early blastula stages. The 

 gastrular movements of many species have now been studied and show 

 that they have achieved a variety of solutions to the problem of establish- 

 ing three tissue layers. Three mechanisms of gastrulation will be dis- 

 cussed: in Amphioxus, in the frog, and in the chick. 



AMPHIOXUS 



Amphioxus or the Lancelet is a primitive chordate, a marine animal 

 resembling the fishes. As seen in Fig. 27a, cleavage is virtually equal and 

 produces a blastula whose vegetal-pole cells are only slightly larger than 

 those at the animal pole. Gastrulation begins when the cells at the vegetal 

 pole move to the interior ( Fig. 28 ) . As they press upward, the blastular 

 cavity grows progressively smaller and finally disappears when the vege- 

 tal-hemisphere cells press up against the animal-hemisphere cells. The 

 embryo is now a double-walled cup such as might be made if we pressed 

 upon one side of a hollow rubber ball. The external wall is the ectoderm 

 and will ultimately give rise to the epidermis, certain external organs, and 

 the nervous system. The internal wall is the endoderm, and the hollow 

 U-shaped cavity is called the arch enter on. Surrounded by endodermal 



