DEVELOPMENT OF EXTERNAL GENITALIA IN THE HUMAN EMBRYO. 71 



As the result of this early work we find, in reviewing the literature on the sub- 

 ject, that various views were held by these early scientists concerning the sex of 

 the younger embryos during the so-called indifferent period. Many of them 

 considered that the younger embryos represented a stage intermediate between 

 males and females. Others, for example Tiedemann (1813), who is conspicuous 

 for the accuracy of his observations, believed that, because so many more females 

 than males were observed, all human embryos were at first female and that the 

 males resulted from an advance in development over this more primitive condition. 

 The subsequent investigations of Joh. Muller (1830), Rathke (1832), and Bischoff 

 (1842) resulted in but minor additions to the observations of Tiedmann. In fact, 

 Muller considered that the latter were so remarkably complete that additional 

 investigations could not be expected to produce anything new. Ecker (1851-59) 

 published some excellent figures of the external genitalia of human embryos, which 

 have served as a basis for the somewhat inaccurate Ecker-Ziegler wax models of 

 this region. No other notable papers were published until about 1889, when the 

 development of modern instruments, the introduction of modern methods of tech- 

 nique, and the consequent improvement in the quality of the preparations, resulted 

 in a renewed interest in all branches of biological study. In the embryology of the 

 human urogenital system this era was heralded by the work of Tourneux (1889) 

 upon a series of 35 specimens ranging in length from 24 mm. to 35 cm. Only 10 of 

 these were less than 40 mm. in length, while the majority were fetuses over 50 mm. 

 in length. Although Tourneux described the external genitalia of the majority 

 of these specimens, several of which are shown by fairly accurate figures, the 

 number of young embryos was relatively so few that his attention was not directed 

 toward possible sex differences in these early stages; hence, more consideration 

 was given to the histological study of the urethral canal and the formation of the 

 prostate. In the same year Nagel published the results of his study of a small 

 series of human embryos. This was also devoted mainly to the internal urogenital 

 system, although Nagel made some observations upon the external genitalia which, 

 in part, predicted some of the results of the present study. He believed, for 

 example, that the urethral groove should furnish a clue to the sex of an embryo at a 

 much earlier period than it had previously been possible to recognize sex, although 

 his observations indicated that the males possess the shorter groove, while my 

 findings tend to show quite the opposite. Nagel furthermore found that the 

 outer genital (labio-scrotal) swellings do not arise as a more or less complete ridge 

 surrounding the phallus, as had previously been figured, but as separate, lateral 

 swellings which only later become connected in front of the phallus in the female, 

 and in the male are shifted to a caudal position to form the scrotum. The classical 

 investigation of Keibel (1896), giving an exhaustive account of the development of 

 the internal urogenital organs, based upon a series of 13 embryos (3 to 25 mm. in 

 length), was probably the first study of this system by the modern method of plate 

 reconstruction whereby it has been possible to correlate the development of the 

 external with the internal organs. While Keibel figures the external genitalia in 

 several of his reconstructions, especially those of embryos 11.5 and 25 mm. long, 





