Experiments on Gastrulating Eggs 19 



De Lange has found evidence of these two growth centres in 

 Megalobatrachus japonicus which quite agrees with what we find 

 in Rana and Hypogeophis. 



De Lange suggests a further term tritogenesis to describe the 

 outer growth of the tail. But clearly the growth of the tail is 

 a simple continuation of the deuterogenetic growth, and it certainly 

 is not anything like so fundamental a change as that from proto- 

 genesis to deuterogenesis. 



Experimental Observations. 



The recognition of these two growth centres in the frog is due 

 to experimental work. It is possible by marking parts of the 

 surface of the gastrulating egg to realize movements which other- 

 wise escape one. 



Observations by marking the surface can be made in various 

 ways. It is possible to cauterise a few cells and the scar thus 

 produced can be traced during a shorter or longer period of time 

 and thus the relative movements of parts determined. 



Or bristles may be inserted, at any rate in the egg of Rana 

 temporaria, with many satisfactory results. A bristle makes an 

 unmistakeable landmark, which cannot be said of the scar made 

 by cautery. In cauterisation it is possible for the cauterised cells 

 to slough off: and to get carried away from the spot which they 

 are intended to mark. The bristle method it may be noted 

 cannot be used with Triton eggs. Then again stains such as Nile 

 blue sulphate may be used, as shown by Goodale, who stained 

 little groups of cells round the equator of the egg of Spelerpes 

 bilineatus shortly before gastrulation. The effect of this treat- 

 ment is to stain the yolk grains so that the cells go on dividing 

 and thus the individual descendants can be followed to a certain 

 extent. Apparently all eggs are not equally suitable for this 

 staining method. But experiments on Spelerpes and Amblystoma 

 were successful. 



It may be said at once that the results of all these methods 

 of experimental research are contrary to the theory of concrescence, 

 whether they are made upon Elasmobranch, Teleostean or 

 Amphibian eggs. 



By the bristle method it is possible in the frog's egg to make 



22 



