176 NUCLEOCYTOPLASMIC RELATIONS 



a Habrobracon into a black dahlia. Superficially, it would appear 

 that the interaction of cells and cellular components that occurs 

 in embryogenesis has its basis in this coded information con- 

 tained in the genetic nucleus. The simplest model would be an 

 evolving system in which the yolk acts as a nutrient medium and 

 the cytoplasm as a transport system that funnels nourishment to 

 the nucleus and utilizes information from it. Inasmuch as de- 

 velopment of such systems as legs, eyes, wings, antennae, the 

 thorax, and abdomen are under control of the genetic nucleus, by 

 the doctrine of obvious hypothesis, the entire differentiation 

 system has its basis in the genetic nucleus as well. That this 

 model is too simple, however, is apparent upon consideration of 

 determinate development in which egg regions are predestined 

 prior to the existence of nuclei in these regions. The egg cyto- 

 plasm guides the totipotent nuclei into specified but not com- 

 pletely fixed channels of development. The final fixation is a 

 nuclear function. The cytoplasm in determinate eggs therefore 

 must be a repository for a different order of informational organ- 

 ization from that of the nucleus. Since regulative and mosaic 

 development are essentially alike (Penners, 1924; Tyler, 1930), 

 a general model for illustrating development should include a 

 directive action of fixed extent in the cytoplasm. 



We are interested in the early development of the egg and 

 embryo because the potential exists here for the differentiated, 

 fully developed organism. Before an egg can be activated, its 

 development must be inhibited; prior to embryonic differentia- 

 tion, the potentialities must exist. By analyzing several compo- 

 nents of the predifferentiation system, perhaps the entire em- 

 bryonic organism will be more clearly understood. As the provi- 

 sional model, the young embryo will be considered as a substrate 

 pool for an itinerant nucleus with reproduction as its sole activity 

 until diverted by a determined cytoplasm. 



General Considerations 



The experimental animal most often referred to is Habrobracon 

 juglandis (Ashmead), a small Braconid wasp that parasitizes the 

 larva of the Mediterranean flour moth Ephestia kiihniella Zeller. 



