48 J. M. ESSENBERG. 



were used successfully. For special stains Mallory's triple stain 

 and Harris' hsemotoyxlin were used. All material except adult 

 ovaries were sectioned in paraffin, the latter in celloidin. Very 

 young fishes were sectioned as a whole, in medium-sized fishes the 

 entire viscera were sectioned, and in the mature fishes the gonads 

 and ducts only were preserved for sectioning. For general study 

 sections were cut 6 //, in thickness. 



The body form of the female is strikingly different from that 

 of the male, which fact can be used to advantage in following the 

 development of the species. To express it in numbers, the length 

 of the fish is divided by its greatest depth. The resulting ratio is 

 known as the form index and can be obtained in two ways, by 

 using either the total length or the body length of the fish as the 

 numerator. The former method is less subject to error in the case 

 of Xiphophonis hclleri and will be used exclusively in this work. 



As the anal fin undergoes a marked post embryonal change in 

 the males in which the third ray increases immensely in diameter, 

 while the fourth and fifth rays, although participating in the trans- 

 formation, suffer very little, if any, increase in diameter, a ratio of 

 the third by the fourth ray suggests itself. This ratio may be 

 called the fin ratio. 



It gives me great pleasure to acknowledge my indebtedness to 

 Prof. F. R. Lillie and Dr. A. W. Bellamy for numerous sugges- 

 tions and criticisms during the progress of this work. I also wish 

 to express my appreciation for the most painstaking services ren- 

 dered by our artist, Mr. K. Toda, our technician, Miss D. Brockett. 

 and our librarian, Miss E. L. Dickinson. 



III. THE INDIFFERENT STAGE. 



At the time the young fishes are born they measure on the aver- 

 age 8 mm. in total length, while sex is unmistakably established at 

 the length of 10 mm. This means that the indifferent stage of 

 sex development is almost entirely pre-natal. The form index is 

 6.26 and the fin ratio is i.o at birth. The gonads, although small, 

 are distinctly set off from the surrounding tissues and lie one on 

 each side of the body cavity, immediately below the air bladder, 

 suspended in a peritoneal sac. The peritoneal sac, which becomes 

 the permanent wall of the gonad, is a protruded portion of the 



