MICHIGAN ACADEMY OF SCIENCE. 123 



THE EFFECTS OF CENTRIFUGAL FORCE UPON THE EMBRYONIC 

 DEVELOPMENT OF SOME CHRYSOMELID BEETLES. ' 



R. W. Hegner. 



Centrifugal force was first employed as a method in experimental em- 

 bryology by Roux in 1S84. The real value of the centrifuge, however, was 

 not fully recognized until a few years ago. It has recently been used to test 

 a number of conclusions reached by morj^hological methods. Of these I 

 may mention the following: Roux (1884). Born (1885), O. Hertwig (1897 + ), 

 Morgan (1902 + ), and Gurwitsch (1904) have subjected the eggs of frogs 

 and toads to centrifugal force of various strengths. Their results seem to 

 show that gravity has no effect upon the embryonic development of these 

 Amphibians, that yolk is heavier than cytoplasm, that strong centrifugal 

 force causes the development of abnormal embryos due probably to the fact 

 that certain parts of the egg are rendered incapable of development, that the 

 pigment does not act as a formative substance, that as an egg advances 

 in development it becomes l^ss susceptible to the influence of centrifugal 

 force, and that protoplasm must be in a fluid condition to allow the yolk 

 granules to be driven through it and still be capable of development. Ex- 

 periments have also been made to learn the specific gravity of the various 

 substances in the egg (Lyon. 1907 in Asterias, Arbacia, Chaetopterus, Cynthia, 

 Phascolosoma and the common garden spider), to trace substances from the 

 fertilized egg into the cleavage cells (Lillie, 1906 in Chaetopterus) , to study 

 the polarity of the egg (Lillie. 1909 in Chaetopterus) and to investigate the 

 determination of sex in Rotifers (Whitney, 1909). 



This paper is based upon the results obtained from a number of experi- 

 ments with eggs of Chrysomelid beetles. Eggs of the willow leaf-eater, 

 Calligrapha m ulti punctata , were used more than any other but eggs of C. 

 bigshyana, C. lunata, Leptinotarsa decemlineata and Lema trilineata were also 

 employed. The eggs of these beetles are always laid with their posterior 

 ends fastened to a leaf of the food plant. They can therefore be definitely 

 oriented. During the experiments they were placed in cavities in a block of 

 paraffin, fastened in the tube of a centrifugal machine and rotated at various 

 speeds usually about 2000 revolutions per minute. They were taken from 

 the machine at the desired time and either fixed at once or allowed to develop. 

 I shall descrilje ])elow the results obtained with eggs in early cleavage stages, 

 with eggs laid by centrifuged beetles and with the embryos which developed 

 from them. A brief review will then be given of the conclusions reached 

 from a study of the effects of centrifugal force upon eggs in various stages 

 of development. 



A description of the structure of a freshly laid egg of C. bigsbyana will 

 be found in another paper published in this report so I need not repeat it 

 here. After an'egg in the early cleavage stage has been centrifuged for an 

 hour or more, three distinct regions are visible, a colorless cap at the outer 

 end, a bright orange region at the inner end and an intermediate zone yellow 

 at the outer end and becoming less brightlv colored as the inner laver is 



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'A detailed account of all the experiments referred to below is now in press (Jour. Exp. Zool ) 



