186 HAROLD H. PLOUGH 



With the final statement of this result the implication of the 

 discussion becomes obvious. High temperature causes a de- 

 finite change in the amount of crossingover. If the point where 

 this change occurs in the ovary can be established, there is strong 

 evidence for the belief that that is the point where crossingover 

 normally occurs. An examination of the ovaries of females soon 

 after hatching therefore becomes extremely important. 



Cytological data 



For a considerable period the author in collaboration with 

 Dr. C. W. Metz has been endeavoring to work out the cytological 

 features of Drosophila melanogaster (seu ampelophila) . A con- 

 siderable amount of material was therefore at hand. Sections 

 of the whole abdomens of females about twelve hours old were 

 selected, and drawings were made of all of the sections of one 

 ovary from each of two different females. Each ovary was 

 made up of five or more egg strings, as is common in insects, and 

 the eggs go through the growth period as they pass from one end 

 to the other. This results in a definite seriation from oogonia 

 at one end to large, mature eggs ready for fertilization at the 

 other. Because of the fact that oogonia are not surrounded by 

 an envelope of follicle cells, 4 it was an easy matter to mark the 

 cysts of oogonia with a cross as the drawings were made. When 

 each of the series was complete the cysts which were not crossed 

 were all oocytes. Each cyst is surrounded by a follicular en- 

 velope and contains sixteen nuclei, one of which is the future 

 egg nucleus and the other fifteen are nurse cells. Beginning 

 with the drawing of the first section each cyst could be traced 

 individually throughout each of the succeeding sections in which 

 it appeared. And all except the first were checked. When 

 this process was carried through the entire series the total num- 

 ber of cysts or eggs which remained blank represented the total 

 number of oocytes in the ovary. The totals recorded in these 

 counts which include, of course, oocytes just past the last oogo- 



4 . . . . and for other reasons which will be discussed in a later paper on 

 the oogenesis in this fly. 



