MICHIGAN ACADEMY OF SCIENCE. 49 



EXPERIMENTAL STUDIES ON THE RELATION BETWEEN THE 



STRUCTURE AND DEVELOPMENT OF THE EGGS OF 



CHRYSOMELID BEETLES. ^ 



EGBERT W. HEGNER. 



The structure and development of the insect egg differ widely from that 

 of the eggs of most other animals. Two of these differences are (1) the 

 fact that most insect eggs are laid and develop in the air instead of in the 

 water, and (2) because of the comparatively immense amount of yolk, early 

 cleavage is superficial instead of complete, partial or discoidal. In a re- 

 port published five years ago (Hegner, 1909b) some of the effects of centri- 

 fugal force upon the structure and development of certain chrysomehd 

 beetles were described. Since that time the same method of experimenta- 

 tion has been employed by the writer for the purpose of determining (1) 

 the relation between the structure and polarity of the egg, and (2) the com- 

 parative importance of the nuclei and other constituents of the egg in de- 

 velopment. 



The eggs of the willow beetles, Calligrapha muUipunctata and C. higshyana, 

 and of the potato beetle, Leptinotarsa decemlineata, have been used exclu- 

 sively in the experiments here to be described. They have proved to be 

 favorable for this work because these beetles are easily reared in the labora- 

 tory and their eggs are thus available in great numbers. Besides this the 

 normal embryology of these beetles is well known. 



In nature the eggs are laid in batches of from twenty to seventy-five and 

 fastened to the under surface of the leaves of the willow or potato plant, 

 at right angles to their longitudinal axis. Shortly before the eggs hatch 

 the markings of the larva within can be seen through the chorion, and with- 

 out exception the posterior end of the embryo coincides with the fixed end 

 of the egg and the anterior end with the free end of the egg* By marking the 

 free end with India ink it has been possible to maintain an accurate orienta- 

 tion of the egg throughout its entire developmental period. It has been 

 shown in a previous communication (Hegner, 1909b) that the surfaces of 

 the egg corresponding to the right and left sides and dorsal and ventral 

 surfaces of the future embryo can also be determined as soon as the eggs 

 are laid. 



Because of the definite orientation of the egg it is possible to perform experi- 

 ments upon certain regions whose history under normal conditions is known. 

 Thus with a hot needle any portion of the cytoplasm of a freshly laid egg 

 or of the blastoderm of an egg twenty-four hours old can be killed and the 

 part of the embryo to which that particular mass of cyto]ilasm or group of 

 cells would have given rise is prevented from taking part in further develop- 

 ment (Hegner, 1911). 



Centrifugal force has been employed as a method in experimental em- 

 bryology with the principal objects of studying the weights, distribution 

 and chemical constitution of the egg substances, the initial structure and 



1 From the Zoological Laboratory of the University of Michigan, Ann Arbor, Michigan, U. S. A. 



