THE DEVELOPMENT OF THE EXTERNAL GENITALIA IN THE 



HUMAN EMBRYO, 



The present investigation is concerned with the development of the external 

 genitalia in the human embryo, the determination of the stage at which sex differ - 

 ences make their appearance, and how early sex can be recognized from these 

 external structures. In addition to the morphological interest in the structural 

 development of the phallus region, its embryology presents several other phases 

 of almost equal importance. Thus the ability to recognize sex at an earlier period 

 than has hitherto been possible is of great practical value to the clinician as well as 

 to the embryologist. At the same time, this point is largely dependent upon the 

 verification of the existence of a definite "indifferent" ("undifferentiated," Pohlman 

 1904) period through which all embryos have been generally believed to pass before 

 assuming their definite male or female characteristics. 



As a result of this study I have found that there is apparently no real indifferent 

 period. On the contrary, the younger embryos show constant differences in the 

 phallus and almost from the earliest differentiation of the genital tubercle (i. e., 

 embryos 14 mm. GL) they can be divided into two groups. This division is based 

 upon the marked difference in the length of the urethral groove upon the caudal 

 slope of the phallus. This seems to be quite constant and without intergradations 

 and can be traced backward from the older embryos, in which sex can be definitely 

 recognized, to the younger stages which heretofore have been included in the so- 

 called "indifferent" period. In one group the urethral groove extends from the 

 base of the phallus practically to its apex, i. e., onto the region of the future glans; 

 these I consider to be males. In the second group the groove is much shorter and 

 terminates some distance below the apex of the phallus i. e., it does not extend 

 onto the region of the future glans; these I consider to be females. 



I wish to extend my sincerest thanks to Dr. G. L. Streeter for his kindness in 

 according me the privilege of examining the extensive collection of human embryos 

 in the Carnegie Laboratory of Embryology, as well as for the great interest he has 

 taken and the many helpful suggestions he has made throughout the course of the 



investigation. 



MATERIAL AND METHODS. 



The material studied in the preparation of the present paper has been limited 

 to the collection of human embryos in the Carnegie Laboratory of Embryology. 

 This includes a large number of specimens which are graded 2 and 3; that is, they are 

 unsuitable for sectioning because of faulty preservation, mutilation, or some other 

 defect; also a considerable number of sectioned embryos up to 50 mm. in length. 

 While perhaps more attention has been paid to the external features of these unsec- 

 tioned embryos, as far as possible this evidence has been verified by a study of 

 sectioned specimens of corresponding sizes, a number of these having been recon- 



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