148 



AN INTRODUCTION TO MODERN GENETICS 



6. Amphibia 



Amphibian eggs are the most fully studied of the vertebrate types, 

 and provide the classic example of the organization centre. Spemann^ 

 showed that the region round the blastopore can, when transplanted 

 into new surroundings in a gastrula, cause those surroundings to 

 develop into part of the embryonic body including neural tube, noto- 

 chord, somites, etc. This active region round the blastopore, which is 



Fig. 72. Organizers in Amphibia. — A. Ventral view of a young gastrula of a 

 newt, showing the blastopore at the top; the dorsal lip of the blastopore from 

 another egg has been grafted to the bottom right. B. Dorsal view of neurula stage, 

 showing the neural folds of the f>ost embryo, with induced embryo on right. 

 C. Neural folds of induced embryo. D. The host and induced embryos at a later 

 stage (sides reversed). 



(After Spemann and H. Mangold.) 



known as the organization centre, is formed in a region of cytoplasm 

 which becomes locahzed at fertilization or just afterwards; there is no 

 evidence that the zygotic nucleus has anything to do with this locahza- 

 tion which seems to be dependent on the interaction between the 

 gradient of yolk-content from the animal to the vegetative pole and 

 another dor so- ventral gradient.^ 



One part of the process by which the organization centre induces the 

 formation of a new embryonic rudiment is concerned with the produc- 

 tion of particular kinds of tissues, such as neural tissue, etc. This 

 process consists of a reaction between the tissue and a substance or 



^ Spemann and H. Mangold 1924. - Dalcq and Pasteels 1937. 



