152 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



the activation of the egg system, and so it is here designed "the activa- 

 tion center," following Huxley and 4eBeer (1934). The Uteral translation 

 "formative center" is ambiguous and may carry unwarranted implica- 

 tions. It is not a visibly distinct part of the egg but can be more or less 

 closely delimited by experiments. It functions by the production of 

 some substance that diffuses forward through the interior of the egg and 

 initiates development. In its absence no true differentiation of the 

 embryo occurs. 



19 



Activaiion Cenier Onsei of 



before acfivation functioning of the 



19 46 54 64hrs.ai21.5°C. 



Differeniiaiion Completed 



Center Germ Band 

 Acfivation "Center 



a b c d e 



Fig. 59. — Graphic diagram of the development of the egg of the damsel fly Platycnemis. 

 (a) 4-nuelei stage with bracket indicating the position of the activation center. (6) 

 256-nucIei stage with curved line indicating the diffusion anteriorly of the product of the 

 activation center, (c) Beginning of cell-aggregation with bracket indicating the position 

 of the differentiation center, (d-e) Formation and completion of the embryo anlage. 

 One egg division equals 24/li. (Modified from Seidel, 1934.) 



The activation center has been most elaborately analyzed in the 

 damsel fly (Platycnemis) by Seidel (1926-1934). In this species (Fig. 

 59) the anterior border of the activation center coincides approximately 

 with the presumptive posterior end of the embryo (about one-ninth the 

 length of the egg from the posterior pole), but its posterior extent is 

 undetermined. The point at which the germ band later invaginates into 

 the yolk marks its locus. Here the functioning of the activation center 

 depends upon an interaction between the cleavage nuclei and the region 

 of this center. When the cleavage nuclei are late in reaching this region 

 because of abnormal distribution, killing of one of the first two cleavage 

 nuclei with ultraviolet light, or constricting the egg and later removing 

 the constriction, the functioning of this center is delayed. Careful com- 



