724 DEVELOPMENT OF THE COMMON PHALANGER, 



of the small intestine, and to the distention of the stomach and 

 intestines with the ingested milk. This feature is well shown in 

 the embryo of 16 mm., greatest length, shown in Plate xxiii., fig. 8. 

 This embryo, though but little longer than the newly-born speci- 

 men, is, owing mainly to the abdominal distention, of much 

 greater bulk. The epitrichial layer is apparently rather thicker, 

 so that the position of the eye can scarcely with certainty be 

 made out. The umbilicus is quite healed, but the scar is still 

 distinct. There is as yet no indication of the pouch or scrotum. 

 The following are some of the principal measurements : — Head 

 length, 7 mm.; snout to root of tail, round the dorsal curve, 28-5 

 ■mm.; tail, 4-5 mm.; arm from elbow to tip of middle digit, 57 

 mm.; le^ from knee to tip of 4th digit, about 3-5 mm.; foot, about 

 2 mm. 



Beyond a general increase in size there is little chani^e in the 

 external appearance of the mammary ftetus for some time. A foetus 

 of 19 mm, greatest length, is shown in Plate xxiv., fig. 9. The 

 two chief points of interest in this embryo are that rudiiBentary 

 claws are now distinctly present on the four toes of the hind foot, 

 and that, whereas at birth and for a short time afterwards, the 

 nipple is gras^Ded by the tongue and upper lip, here and in all 

 the later stages, the tongue is entirely within the mouth, and the 

 nipple is grasped between the upper and lower lips The umbili- 

 cal scar is still apparent, but as yet there is no evidence externally 

 of the sex. The principal measurements of this embryo are : — 

 Head, 7o mm.; snout to root of tail, round the dorsal curve, 32 

 mm.; tail, 5-5 mm.; arm, 6-5 mm.; leg, 4-3 mm.; foot, 2-5 mm. 



Shortly after this stage important changes begin to be mani- 

 fested in the external characters. With the relatively rapid 

 growth of the hind limb flexion takes place at the knee and 

 ankle, clearly differentiating the parts of the limb. But a more 

 important character is the manifestation of sex by the develop- 

 ment of the scrotum in the male, and of the pouch in the female. 

 In both male and female embryos the sexual characters become 

 manifested about the same time, viz., when the embryo attains to 

 23 mm. in greatest length. Doubtless, microscopic examination 



