and Deuterogenesis 7 



about the fate of the blastoderm just in front of this knob? 

 Does not that give rise to part of the embryo? There can be 

 no doubt about the answer. 



All parts of the blastoderm of a Teleostean or an Elasmobranch 

 become parts of the embryo. 



Thus we have a part of the embryo formed from the layer of 

 cells which exists as a result of segmentation, and another part 

 which comes into being by the activity of the germ rim whether 

 by concrescence or not. 



That is to say we have two definite and in a sense independent 

 centres of growth, one of these comes into being as the result of 

 the fertilisation of the egg, and the other comes into being 

 subsequently. 



These may be termed the primary growth centre and the 

 secondary growth centre or to use terms which I suggested some 

 years ago, the protogenetic and deuterogenetic growth centres. The 

 activities of these centres may be called protogenesis and 

 deuterogenesis respectively. 



In order to understand the action of these centres one must 

 have a clear idea of the formation of the germ layers and of the 

 process and meaning of gastrulation. 



I propose therefore to describe in some detail the formation 

 of the germ layers in the Amphibia. 



Rana temporaria. 



An egg of Rana temporaria is pigmented to a greater or less 

 extent. There is much variation, in some the lower hemisphere 

 is almost quite devoid of pigment, in others it is pigmented but 

 is slightly lighter near the lower pole. 



The whiter pole is the more heavily laden with yolk and it 

 floats downwards in water. To all appearances the egg is radially 

 symmetrical. Soon after fertilisation a curious crescentic whiten- 

 ing of part of the pigmented area can be noticed, this is called the 

 " grey crescent." 



It is said 



(1) That this is always immediately opposite to the point 

 at which the spermatozoon has entered; 



(2) That the future sagittnl plane of the animal coincides 



