THE EARLY DEVELOPMENT OF THE MAMMAL 415 



month. Here we find a pair of ridges, either side of the cloacal 

 membrane (Fig. 173, A), which gradually fuse and enlarge 

 anteriorly, forming, toward the close of this month, a distinct 

 cloacal tubercle. The urinogenital membrane is perforated 



FIG. 173. The development of the external genitalia. A, After Keibel. 

 B-E, After Felix, from Meyer. A. Model of the cloacal region of a human 

 embryo of 3 mm. B. Ventral viW of the caudal end of a human embryo of 

 18mm. C. Same of 28 mm. Indifferent stage. D. Same of 32.5 mm. Female. 

 E. Same of three and one-half months. Male, a, Anal opening; g, genital ridge; 

 gc, glans clitoridis; gp, glans penis; h, hind-limb; Im, labia majora; m, cloacal 

 membrane; ms, median scrotal rudiment; p, phallus; r, cloacal ridge; s, scrotal 

 ridge; t, coccygeal tubercle; u, umbilical cord (in A, umbilicus;) ug, urinogenital 

 aperture. 



about this time by the urinogenital aperture. (The time at 

 which the anal opening is formed is quite variable, but usually 

 is also toward the close of the second month.) 



Upon the cloacal tubercle, and toward its posterior or anal 

 side, there grows out quite rapidly a definitely circumscribed 

 process called, at this stage, the phallus (Fig. 173, B}. The 



