70 



THZ; REALISATION OF THE NUCLEAR FACTORS 



from the rest of the egg by means of a tight ligature, no 

 embryo will develop in the anterior part of the egg (Fig. 

 26 a-d). The activating centre does not begin its activity, how- 

 ever, until one of the cleavage nuclei has reached it. Cleavage 

 in insects differs from that in other animals in that the zygote 

 nucleus, which is located in the middle of the egg, divides 

 repeatedly without accompanying cell-divisions. Therefore, the 

 cleavage nuclei can spread freely throughout the egg. It is only 



Fig. 26. The activating centre in Platycnemis pennipes, and its 



effect, (a) constriction of a very small part of the caudal end of the 

 egg does not prevent the development of the primordium of an 

 embryo (K), from which a normal embryo will later develop (b); 

 (c) constriction of a larger part prevents the formation of an 

 embryo (d); (e) incomplete ligature prevents the cleavage nuclei 

 (black dots) from reaching the activating centre; again, no embryo 

 is formed in this case. After Seidel. 



when a large number of these nuclei has been formed that they 

 migrate to the superficial layer of the egg cytoplasm. Then each 

 nucleus, with the surrounding cytoplasm, is partitioned off 

 from the remainder of the egg to form a separate cell (so- 

 called ''superficial cleavage''). Now Seidel constricted the eggs 

 just anteriorly to the activating centre so tightly that the 

 cleavage nuclei were unable to reach the centre through the 

 narrow passage that remained. In that case no further devel- 

 opment took place (Fig. 26e). But if one nucleus manages to 

 reach the activating centre, a substance is produced there 

 whieh diffuses into the anterior regions of the egg, and there 



