196 GEORGE W. TANNREUTHER. 



instead are produced later. The question of sex determination 

 from the standpoint of external conditions is rather an intricate 

 problem to attempt to solve, since the same male-producing 

 females are capable of producing simultaneously both males 

 and female-producing females (from thin-shelled resting eggs) 

 under the same food conditions. 



COMPARATIVE STUDY OF MALE AND FEMALE. 



The males with few exceptions as found in the different groups 

 of rotifers are smaller than the females. The cleavage and early 

 development in both sexes are quite similar. Previous investi- 

 gators have based their results almost entirely on the study and 

 development of the females and have not made a careful study 

 of the degree of development and degeneration as it occurs in the 

 male, more especially when compared with the conditions found 

 in the female. 



The development of the parthenogenetic female is represented 

 in the series of diagram A, 1-15, and the parthenogenetic de- 

 veloping male in the series of diagrams B and C, 1-15. In 

 case of the male the series B represents the condition, where 

 degeneration of the male individual is very pronounced. Series 

 C represents the condition of normal development, w r here only 

 partial degeneration of the male occurs. 



A. FORMATION AND MATURATION OF OVA. 



The ovary of female-producing male individuals and resting 

 eggs and of the female-producing males are quite similar (Figs. 

 i and 2). It is composed of a group of very small cells (ova), 

 situated on the convex side of the vitellarium, near the point 

 where the oviduct takes its origin. These small cells are uniform 

 in size, and at regular intervals one of them begins its growth 

 and becomes the mature ovum. Occasionally two begin their 

 growth simultaneously. The growth is very rapid and the cyto- 

 plasmic granules pass directly from the vitellarium into the 

 growing egg (Figs. I and 2). When growth is complete the egg 

 is separated from the ovary and passed into the upper end of 

 the oviduct where maturation occurs. The origin and formation 

 of the male, female and resting eggs are quite similar. The rest- 



