144 



AN INTRODUCTION TO MODERN GENETICS 



mosaic, localized injuries to the eggsof Musca result in corresponding 

 local injuries in the larvae. Musca belongs to the group of insects whose 

 cleavage follows a fairly definite pattern. Recently work has been done 

 on other insects whose early development is more indeterminate. 

 Seidel, working on the dragonfly Platycnemis, has elucidated a series 

 of phenomena which may occur quite widely in insect development. 

 The first event of importance, after fertilization, is the arrival of one of 



Fig. 70. The Activation and Differentiation Centres in Insects. — A. A con- 

 striction at the most posterior end of the egg, behind the activation centre, 

 allows a norma! embryo to develop. B. If a constriction is made at an early stage 

 just in front of the activation centre, no embryo develops, the anterior part of the 

 egg forming extra-embryonic blastoderm as in C. D. If the same constriction is 

 made at a later stage, after the activation centre has finished its action, an embryo 

 develops in that part which is still in cellular contact with the differentiation 

 centre. 



(From Seidel.) 



the cleavage nuclei, accompanied by a small quantity of cytoplasm, in 

 the posterior end of the yolk-filled egg. The movement towards this 

 region takes place under the influence of a general repulsion between 

 the cleavage nuclei, and by various means it is possible to make different 

 members of the group of early nuclei arrive first at the posterior end. 

 Any nucleus, no matter which, when it arrives at the posterior end, 

 reacts with the particular cytoplasm locahzed there and sets free a 

 substance which diffuses forward throughout the entire egg. In any 

 region which this substance is prevented from reaching, no develop- 

 ment occurs ; but the substance merely activates development and does 

 not determine the nature of the development of the different regions. 

 Again there is no evidence that the particular genetic nature of the 



