2 HARLEY N. GOULD 



INTRODUCTION 



It has been shown by various authors whose work will be briefly 

 reviewed, that protandric hermaphroditism is common in that 

 group of Prosobranch Molluscs known as the Calyptraeidae, the 

 animals functioning as males during the early part of their lives 

 and as females during the later part. The male and female 

 phases are connected by a transitional period. It was sug- 

 gested to the writer by Prof. E. G. Conklin in 1913 that if a de- 

 tailed histological study were made of the development and 

 history of the gonad in these protandric hermaphrodites, new 

 facts might come to light bearing on the early differentiation of 

 the male and female germ cells, and perhaps on other questions 

 relating to hermaphroditism. Those curious structures found 

 in the testis of so many Prosobranchs, known as the atypical 

 speraiatozoa, are also of interest in this connection. The prob- 

 lem has opened up even more possibilities than could have been 

 anticipated, and I take this occasion to express my thanks to 

 Professor Conklin for his suggestions and constant oversight of 

 the work. 



There are three species of the genus Crepidula, hsted among 

 the Calyptraeidae, obtainable along the Atlantic Coast; viz., C. 

 plana, C. convexa and C. fornicata. The two last show some 

 overlapping of the male upon the female phase, while in C. plana 

 no resting oocytes are normally found in the gonad until the 

 production of spermatozoa has entirely ceased. Upon C. plana 

 the work described in this paper has been chiefly concentrated. 

 The choice of this form for investigation was finally determined 

 by the discovery that it possessed a most striking peculiarity, 

 the dependence of the male phase upon conditions of the envir- 

 onment. The instability of the male phase wifl be discussed in 

 a report shortly to follow the present paper; experiments will be 

 recorded there to show how the male characters may appear 

 and disappear as the environment is changed. 



