52 SIXTEENTH REPORT. 



respects, the principal difference being due to the fact that the yolk and 

 gray-cap substances Avere practically all thrown to the centrifugal end, 

 whereas the lighter oil globules gathered at the centripetal pole, and the 

 embryo developed between these two masses. 



Superficial views of three of these embryos are represented in Figs. 2, 3, 

 and 4. In normally developed eggs the embryo develops around the yolk 

 and at the expense of this nutritive substance (Fig. 5). In these centrifuged 

 eggs, however, the yolk has been thrown away from the cytoplasm and a 

 dwarf embrA'o has resulted in ever}^ case. These dwarfs resemble in general 

 the normal embryos in appearance but present many minor abnormalities. 

 The head and thorax seem to have been less affected than the abdomen, 

 probably because in normal development the tail-fold is pushed around on 

 the dorsal surface of the yolk (Fig. 6) and later contracts until its posterior 

 end coincides with the posterior end of the egg (Fig. 5). The shifting of 

 the 3^olk would therefore seriously affect the growth of the tail-fold. 



Perhaps the most interesting feature of these dwarf embr^yos is the fact 

 that their orientation is as nearly like that of the normally developed em- 

 bryo as the mechanical conditions would allow. Thus while they lie more or 

 less transversely in the egg the head end is directed toward the anterior 

 pole of the egg. 



Conclusion. The original polarity of the beetle's egg is retained in the 

 cytoplasm in spite of the shifting of the yolk, gray-cap material, and cyto- 

 plasm by centrifugal force. 



Discussion. The roles played by the nucleus and cj^toplasm in heredity 

 has for many years furnished interesting problems which have been at- 

 tacked by both morphological and experimental investigations. The litera- 

 ture of the subject is extensive and no attempt will be made to review it 

 here, but the relation of the results of the experiments described in the 

 preceding pages to the ideas that are now prevalent may be discussed to 

 advantage. 



Studies on the organization of animal eggs have shown that the cyto- 

 plasm of the egg is not a homogeneous substance, but is heterogeneous. 

 The chromatin has for many years been considered the special substance 

 containing the determiners of hereditary characteristics. Experiments, such 

 as the fertilization of the egg of one species of animal by the spermatozoon 

 of another species, have shown that the large features in development, that 

 is, the features of the phjdum, class, or order, are controlled by the cytoplasm 

 since they appear before the male and female nuclei exert any influence on 

 embrj'onic growth. These characters must therefore be transmitted by 

 the cytoplasm of the egg and more is therefore inherited from the egg than 

 from the spermatozoon, and hence more from the mother than from the 

 father. The smaller characteristics, that is, those of a generic, specific, 

 varietal, or individual nature, appear later and are influenced by both the 

 male and female cells, presumably being largely under the control of the 

 nuclear substance, chromatin. The differentiations leading to the modifi- 

 cation of the larger characters by the chromatin must take place in the 

 cytoplasm, and since the cytoplasm is known to be heterogenous the object 

 of many investigations has been to discover the factors of differentiation 

 that exist in the egg substance. 



The beetle's egg consists of a large central mass of yolk surrounded by a 

 thin superficial layer of cytoplasm (Fig. 7). The maturation divisions of 

 the egg nucleus take place in the cytoplasm near one side of the egg. The 



