XXXVIU GENERAL INTKODUC'I'lON. 



lioi'se, during tlie first six weeks, is nourished like the 

 yoaug opossum and kangaroo by " uterine milk " filtering 

 into the yolk-sac; but beyond this period the fcetal blood 

 is brought into close relation with that of its dam, and thas 

 a fresh food-supply is tapped. Daring the seventh week, 

 while the new machinery is being fitted up, the embryo, 

 uncertain as to its fate, behaves like a young opossum 

 when preparing for its life in the poach. Few now sup- 

 pose that young opossums, wallabies (rock kangaroos), 

 and other Marsupials reach the pouch by crawling down 

 the teats. On the other hand, few seem to know how the 

 young wallaby, e. </., manages to glue itself so firmly to 

 the teat as soon as it reaches the pouch. At the end of 

 each teat (as Mr. le Souef of ^lelbourne recently explained 

 to me) there projects a delicate tube. The female wal- 

 laby conveys the young as they are born by means of her 

 lips to the jioncli, which is kept open during the process 

 by her short fore-limbs. Even at this early period the 

 lips of the young, when stimulated, may contract ; at any 

 rate, the minute tube hanging from the end of the teat 

 gets into the mouth of the helpless youngster, and is 

 probably held in position for a time by the lips. The 

 lips, however, if active at this stage, would soon get ex- 

 hausted. I'o prevent the young one slipping, the slender 

 tube at the end of the teat is distended, it is dilated so 

 as to fill the mouth, the result being that the young are so 

 firmly fixed that a considerable force is required to drag 

 them from their moorings, and when once removed it is 

 impossible to replace them. The dead or helpless embryos, 

 sometimes found lying in the pouch, have probably either 

 slipped from the lips of the mother or dropped from the 

 teat before the delicate terminal tube was inflated. In 

 the young Marsupial, it may be mentioned, the lips are 

 relieved from the Imsiness of sucking ; the milk is pnmijed 

 into their mouths by certain muscles compressing the 

 mammary glands. 



It may seem a far cry from a new-born wallabv to a 

 seven-weeks horse embryo, but the distance may not be 

 so great as it seems. Not only is the lower iaw in a 



