DEVELOPMENT OF EXTERNAL GENITALIA IN THE HUMAN EMBRYO. 



73 



the external genitalia is rather meager as compared to that of the internal organs. 

 Certain of my observations have led me to conclusions different from those reached 

 by Felix; the discussion of these moot points will be taken up in the section describ- 

 ing the development of the external genitalia. 



STAGES OF DEVELOPMENT OF THE EXTERNAL GENITALIA. 



While the greater part of the investigation has been devoted to a study of the 

 changes occurring after the embryos have attained a length of 14 mm., a few 

 observations have been made upon younger specimens which are here included as 

 forming a starting-point for the present description. 



For the sake of convenience, the sequence of development of the external 

 genitalia has been divided into three periods, each of which is represented by one or 

 more stages: (1) genital-tubercle period, characterized by the more or less conical 

 form of the genital eminence prior to the formation of the labio-scrotal swellings; (2) 

 phallus period, beginning with the definite appearance of the labio-scrotal swellings 

 which separate the conico-cylindrical outgrowth of the genital tubercle from the 

 surrounding body areas; and (3) definitive period, characterized (particularly in 

 male embryos) by the transition of the primary external genitalia into what is 

 essentially their final form. 



CLANS 



EPITHELIAL TAG 



m 



CORONARY SULCUS 



SHAFT 

 -UROCENITAL OPENING 



^H/t URETHRAL FOLDS r ^ 



LATERAL PHALLIC GROOVED 

 LAB1O.SCROTAL SWELLINGS 



ANAL TUBERCLES 



RAPHE 

 POSTERIOR COMMISSURE 



B 



TEXT-FIGURE 2. Drawings illustrating the parts of the external genitalia at the beginning of the definitive period. 

 A. male embryo 45 mm. long (Specimen No. 948); B, female embryo 49 mm. (Specimen No. 2256). X 20. 



GENITAL-TUBERCLE PERIOD. 



Stage 1, 9 mm. (fig. 1, plate 1). The earliest stage that I have studied is shown 

 in an embryo 9 mm. long. In this specimen the genital tubercle is a very low 

 conical eminence between the umbilical cord and the base of the tail. Its apex is 



