40 EARLY AMPHIBIAN DEVELOPMENT 



Sauropsida, by means of centrifugalisation. The eggs orientate 

 themselves in the centrifuge tube in such a way that the animal pole 

 is directed centripetally, and the yolk is concentrated into an 

 abnormally dense mass at the vegetative pole. Cleavage then results 

 in the formation of a disc of cells or blastoderm resting upon an 

 undivided mass of yolk. The nuclei of some of the blastomeres 

 migrate into the yolk and become enlarged, irregular and 

 chromatic, and thus resemble the "yolk-nuclei " (bodies responsible 

 for the precocious digestion of the yolk) characteristic of selachian 

 development (fig. 12).^ 



The causes of cleavage concern the problem of cell-division, 

 which, as such, lies outside the scope of this book. 



Modified cleavage of frog's egg, under the influence of centrifugal force. The 

 yolk (d) is concentrated in the vegetative hemisphere, and cleavage results in the 

 formation of a blastoderm, m, yolk-nuclei ; kh, blastocoel. (After Hertwig, from 

 Jenkinson, Experimental Embryology, 1909-) 



§4 



Following upon cleavage, the next step is gastrulation. This process, 

 which, of course, results in the conversion of a single-layered hollow 

 ball (the blastula) into a double-layered sac (the gastrula), is 

 heralded in Amphibia by the appearance of the dorsal lip of the 

 blastopore at a particular latitudinal level on the blastula, in the 

 dorsal meridian. The level at which the lip appears is under the 

 control of the primary physiological gradient along the egg-axis, 



^ Hertwig, 1897, 1904; Jenkinson, 191 5. 



