412 ROBEET W. HEGNER 



of the cytoplasm in which they happened to become imbedded. 

 The writer has shown (Hegner, '11a) that if the posterior end of 

 a freshly laid egg of Leptinotarsa decemlineata is killed with a 

 hot needle, thus preventing the pole-disc granules and surround- 

 ing cytoplasm from taking part in development, no primordial 

 germ cells will be produced. A large series of similar experi- 

 ments have also proved that at the time of deposition, "The 

 areas of the peripheral layer of cytoplasm (fig. 7, A, khhl.) are 

 already set aside for the production of particular parts of the 

 embryo, and if the areas are killed, the parts of the embryo to 

 which they were destined to give rise will not appear. Like- 

 wise, areas of the blastoderm are destined to produce certain 

 particular parts of the embryo" (Hegner '11a, p. 251). What 

 becomes of the nuclei which are prevented from entering the 

 injured region of the egg? No evidence has been discovered to 

 indicate that they disintegrate, so they probably take part in 

 development after becoming associated with some other part 

 of the egg. If these nuclei were qualitatively different they 

 should produce germ cells and other varieties of cells in what- 

 ever region they chance to reach. It is evident that they are 

 not potentially different and that their 'prospective potency' 

 and 'prospective significance' do not coincide. The cytoplasm 

 is, therefore, the controlling factor at this stage in the germ-cell 

 cycle, although cytoplasmic differentiations are for the most 

 part invisible and probably the result of nuclear activity during 

 earlier stages. 



Amitotic nuclear division in vitellophags. The cleavage nuclei 

 in Chrysomelid eggs all divide by mitosis, but, after the blasto- 

 derm has become estabhshed, the vitellophag nuclei, which 

 remain behind in the yolk, show evidences of amitotic division. 

 At this stage of development the vitellophags are more or less 

 evenly distributed within the yolk mass. Here and there groups 

 of from two to four or more nuclei are present, indicating that 

 several divisions have occurred in quick succession. All of the 

 stages characteristic of amitotic nuclear division are abundant. 

 Three of them are shown in figure 56; nucleus a is just becoming 

 dumb-bell-shaped; nucleus h has almost become constricted into 



