152 AN INTRODUCTION TO MODERN GENETICS 



specific nature of the reacting tissue; (3) the specific nature of an 

 organizer affects the pattern in which the induced tissues are arranged, 

 but this pattern is also influenced by the specific nature of the reacting 

 tissues. In all these processes, the specific constitution of the tissues 

 seems to be dependent on cytoplasmic as well as nuclear factors. 



7. Other Vertebrates 



Organizers have been shown to exist in all other classes of vertebrates 

 except the reptiles, which present technical difficulties which have not 

 yet been overcome. The mechanism of their action is probably the 

 same in principle as that of the amphibian organizers, namely the 

 production of new types of tissue by the action of evocator substances 

 acting on competent tissues, with some degree of control by the organ- 

 izer of the way in which these new tissues are arranged. The position of 

 the main organization centre within the embryo differs, of course, in 

 different classes. It is always related to the focus of the gastrulation 

 movements. Thus in fish^ the endoderm and mesoderm are both formed 

 at the posterior edge of the embryonic disc, and this region is the 

 organizer. In birds,^ the gastrulation is split up into two phases, and so 

 is the organizer. In the first phase, the endoderm is formed in the 

 posterior region of the blastoderm and the newly formed endoderm 

 is an organizer which induces the next stage of gastrulation, which is 

 marked by the appearance of the primitive streak from which the 

 mesoderm is given off. The region in which the mesoderm is being 

 most actively invaginated is then in its turn a second organization 

 centre and induces the formation of the neural plate. In mammals,^ it 

 has been shown that the primitive streak can induce neural tissue 

 (when transplanted into a chick blastoderm) but nothing is known of 

 the organizing powers of the endoderm; the technical difficulties of 

 testing it are very great. 



8. Acetabularia 



A imique opportunity for investigating the role of the nucleus in 

 development occurs in the rather special case of the unicellular green 

 alga Acetabularia mediterranea. In spite of the fact that this plant 

 consists of only a single cell, with a single nucleus, it attains a consider- 

 able size, up to about 5 cm. In shape it is rather like a toadstool, with 

 a long narrow stalk bearing at one end an umbrella-like "hat" and at 



1 Luther 1935, Oppenheimer 1936. 



- Waddington 7932, 1933. ^ Waddington 1937. 



