MICHIGAN ACADEMY OF SCIENCE. 53 



female pronucleus, surrounded by a small amount of cytoplasm, then 

 moves toward the center of the yolk mass forming a sort of minute island. 

 Here the male pronucleus unites with it and the first cleavage divisions 

 occur. The cleavage nuclei migrate toward the periphery as they increase 

 in number (Fig. 8), finally fusing with the superficial layer of cytoplasm. 

 The blastoderm of a single layer of cells is thus formed (Fig. 9). 



The visible substances within the freshly laid egg are (1) the superficial 

 layer of cytoplasm, (2) yolk globules of various sizes, (3) the male and female 

 pronuclei, and (4) a mass of granules near the posterior end which take 

 part in the formation of the primordial germ cells and which I have for this 

 reason called "germ-track-determinants." 



The gray-cap which appears at the heavy end of the egg after centrifug- 

 ing must have been distributed throughout the yolk mass of the normal 

 egg, and is probably nutritive material of some kind since the experiments 

 show that it is not necessary for the normal development of the egg. 



When the position of the fundamental egg substance, that is, the super- 

 ficial layer of cytoplasm, is not changed by the centrifugal force, develop- 

 ment proceeds in the normal fashion. This proves that the shifting of the 

 yolk and the gray-cap material does not have any decided influence upon 

 the regular growth processes. When this cytoplasm is displaced by a stronger 

 force and the yolk is driven entirely to one end, the embryo develops near 

 the opposite, lighter end independently of the j'olk. That no conspicuous 

 changes occur in the formation of such an embrj'o indicates that the cyto- 

 plasm in its new position retains the potentialities it possessed in its normal 

 environment. 



A morphological study of the embryology of chrysomelid beetles led the 

 writer some years ago (Hegner, 1909a) to the conclusion that the nuclei 

 that enter the superficial layer of cytoplasm just before the blastoderm is 

 formed are all alike potentially but that the cytoplasm is heterogeneous 

 and controls the future history of any particular blastodermic area. Later 

 (Hegner, 1911) by the experimental method of killing parts of the egg mth 

 a hot needle it was found that the killing of a portion of the cytoplasm of 

 the fresh egg resulted in the absence of the part of the embryo to which 

 this cytoplasm would otherwise have given rise, although no nuclei were 

 destroyed. 



Conclusion. Three lines of evidence lead to the conclusion that in the 

 early development of chrysomelid eggs the superficial layer of cj^toplasm, 

 the "Keimhautblastem" of Weismann, is the controlling substance and 

 that this cytoplasm is heterogeneous. These fines of evidence are (1) the 

 morphological evidence derived from the study of the normal embryology, 

 (2) the experimental evidence obtained by killing parts of the egg, and (3) 

 the experimental evidence procured by shifting the egg substances with 

 centrifugal force. Thus far,, however, no visible differentiations have been 

 observed in the cytoplasm except at the posterior end where the gerra- 

 track-determinants are in some way concerned in the production of the 

 primordial germ cells. 



Ann Arbor, Michigan, 



November 12, 1914. 



