180 



THE AMERICAN NATURALIST [VoL.LV 



fresh developer for each plate. In the ease of the first slide 

 stained in alnmcarniine and Bleu de Lyon as in the Set A, the 

 normal exposure for the 10 micromillimeter section was twice 

 that for the 5 one, and the normal exposure for the 20 micromilli- 

 meter section w^as four times that of the 5 one. In the case of 

 the Set B all slides required the same exposure. 



In analyzing the results thus obtained we come to the con- 

 clusion that the thickness of the section, within the limits given, 

 has no influence whatsoever on the length of exposure ; but that 

 the latter stands in a direct ratio to the amount of stain absorbed 

 by the tissue. 



For practical purposes, esipeeially for those who are using my 

 table of E-P factors, the results of this investigation may be 

 interpreted as follows: disregard the thickness of the sections 

 and the appearance of the stain under microscope and pay at- 

 tention only to the intensity as it appears to the naked eye. The 

 table refers to normally well stained sections of medium thick- 

 ness. Double the exposure for darker appearing and reduce the 

 exposure by half for lighter appearing sections. 



It will be observed that in the experiments elements 2 and 3 

 remained of constant value for the simple reason that the tissue 

 used was not only the same in kind, but actually from the same 

 piece of organ and was stained in the same stain in each set of 

 slides. A comparison of the length of normal exposure in the 

 case of sections stained in alumcarmine and Bleu d-e Lyon with 

 those stained in hematoxylin and tetrabromfluorescie acid serves 

 to confirm my statement at the beginning of this article, that the 

 relative luminosity of different stains may be entirely disre- 

 garded in the matter of exposure. 



ALEX.VNDER PeTRUNKEVITCH 



SEX RATIOS IN PLATYGASTER 



Ix tlio JourHol of Heredity for last year (Vol. X, p. 344) I 

 puhlisln'.l data on sex ratios in three species'of polyenibrj^onic 

 irymcnopToi'a. One of these is Flatygaster felti, which para- 

 sitizr-s the egtrs of two sp(H-ies of tlie Ceeidomyiidjc. Walshomijia 

 taxana and RhopaJomi/ia ftahhur. Tliese flies make their galls 

 on the mountain crdar. S(ih!„a sahi nodes. 



Since the above data uvrc liuWislied I have had an oppor- 

 tunity to breed out in the laboraton- a total of 200 broods of 



