50 SIXTEENTH REPORT. 



polarity of the egg. the constitution of the mitotic figure, the significance of 

 organ-forming substances, and as a general suljstitute for the force of 

 gravity. 



In my earlier report I pointed out the fact that the development of the 

 insect egg is not only uninfluenced bj' gravity but is not even altered by a 

 centrifugal force many times stronger than the force of gravity. The larvae 

 that hatched from eggs that had been subjected to a moderate centrifugal 

 force appeared to be perfectly normal, and many of them were reared in 

 the laboratorv until they pupated. Normal adults emerged from the pupae. 

 There can be no question therefore regarding the abilit}' of the beetle's egg 

 to develop normally after centrifuging. In most cases where centrifugal 

 force has been used in experimental embryology the late embrj^onic and 

 post-embryonic stages have not been studied and hence whether they were 

 normal or not remained undetermined. 



A very strong centrifugal force for a short period or a weaker force for a 

 longer period affect the beetle's egg quite differently. As has been recorded 

 for many kinds of eggs, both of vertebrates and invertebrates, the egg sub- 

 stances were found to be distributed by a strong centrifugal force in three 

 distinct strata according to their specific gravities. The beetles' eggs were 

 placed in cavities in a paraffin block so that a longitudinal position while 

 in the centrifuge was maintained. Eggs centrifuged with either the pos- 

 terior or anterior end toward the center of rotation exhibited three zones: 

 (1) a bright-orange light zone at the inner end which I have called the vesi- 

 cular zone (Fig. 2, v. z.), (2) a comparatively large central mass composed 

 of yolk globules which are largest at the outer heavy end, gradually Ijecom- 

 ing smaller until almost indistinguishable from cytoplasm at the inner 

 end (Fig. 2, y), and (3) a colorless laj^er, which I have called the gray cap, 

 at the extreme heavy end (Fig. 2, g. 1). 



Experiments to determine whether the material of the gray cap is necessary 

 for normal development. 



The material of the gra}^ cap is the heaviest of the egg contents. It is 

 composed of very fine colorless granules whose positions before being driven 

 to the heavy end of the egg could not be determined. Sections through 

 eggs that were allowed to develop after being centrifuged showed the gray 

 cap still at the heavy end. This led to the conclusion that the substance of 

 the gray cap is not necessary for the normal development of the embrN-o. 



TABLE I. 



Eggs of Leptinotarsa decemlineata, laid at 4 P. M., June 11 : centrifuged for one hour when IS hours 

 old. 



A. Control No. 1 



B. Control No. 2 

 C. 



D. 



6 eggs not centrifuged Hatched June 16. 



6 eggs centrifuged Hatched June 16. 



9 eggs centrifuged, gray cap at posterior end killed. . One hatclied June 16: eight 



developed but did not 

 hatch. 



8 eggs centrifuged, gray cap at anterior end killetl.. One developed to hatching 



stage; seven abnormal. 



To test this conclusion the following series of experiments were performed 

 (Table I). Eggs that had been allowed to develop for eighteen hours were 

 centrifuged for one hour, some with the anterior eiitl and others with the 

 posterior end toward the center of rotation. When these eggs were taken 

 from the centrifuge they were placed in watch glasses and the gray-cap 



