BY R. BROOM. i'lO 



will reveal the diiferent characters at a considerably earlier stajie, 

 but while in an embryo of 20 mm. or even 22 mm. it is impossible 

 to tell the sex from external examination, in an embryo of 23 

 mm. this can usually be told, and there is not the slightest diffi- 

 culty in any of the later stages. 



A beautifully preserved male embryo of 23"8 mm., greatest 

 length, is shown in Plate xxiv., fig. 10. In most respects this 

 embrvo resembles the earlier stage fiajured, but the distinct 

 advance can be seen in the further development of the hind limbs. 

 The umbilical scar can still be made out, but is well covered with 

 epithelium. In the lower part of the abdomen and extending across 

 the middle line is a somewhat indistinct bilobed slight elevation. 

 This is the earliest external manifestation of the scrotum. The 

 following are the principal measurements at this stage :— Head, 

 9'5 mm.; snout to tail, 38 mm.; tail, 6 mm.; arm, 8 mm.; leg, 5 

 mm.; foot, 3 5 mm. 



In the female embrj^o the pouch begins to appear as a median 

 depi-ession in the lower part of the abdominal region. 



Plate xxiv., fig. 11, represents a female embryo of 27 mm., 

 greatest length, with the nipple still retained in the mouth. The 

 pouch is well marked, and the clitoris is large and still indistinguish- 

 able externally from the penis in the male. The pinna of the ear is 

 beginning to free itself from the epitrichial layer. In addition 

 to the claws on four toes of the hind foot, a rudimentary claw 

 can now be detected on the hallux. There are as yet no external 

 indications of hair. The following are the principal measure- 

 ments : — Head, 10 mm.; snout to tail, 41-5 mm.; tail, 7 mm.; 

 arm, 10 mm.; leg, 6 mm.; foot, 3-8 mm. 



In an embr3'0 of 30 mm., greatest length, hair has already 

 appeared on the sides of the head, along the shoulders, down the 

 arms as far as the elbows, and along the sides of the body to the 

 lower part of the chest. The pinna of the eai-, though still 

 closely applied to the side of the head, is practically free. The 

 principal measurements at this stage are : — Head, 12 mm.; snout 

 to tail, 47 mm.; tail, 8-5 mm.; arm, 11-5 mm.; leg, 7"5 mm.; foot, 

 4 mm. 



