108 MAMMARY APPARATUS OF THE MAMMALIA 



We have seen that in the Monotremes as well 

 as in the Marsupials, the primary-primordia 

 originate in a dorso-lateral position, and that 

 with the disappearance of the membrana re- 

 uniens inferior they shift towards each other 

 in a medio- ventral direction to take up their 

 final abdominal position. Exactly the same 

 ventral shifting is shown by the milk-streaks 

 and milk-lines of the Placentalia. Further, 

 in all three orders of Mammals, the skin- 

 muscle plate of the early embryo reaches 

 invariably as far as the cutis thickenings below 

 the first primordia of the mammary apparatus, 

 whether they be primary-primordia or milk- 

 streaks. 



It is true that in Echidna and in those 

 marsupial species investigated so far, the 

 primary-primordia form areas of quite small 

 extent, situated on the caudal part of the 

 abdomen, and onlv 3-5 times longer than 

 broad. The milk-streaks, on the other hand, 

 typically extend over the whole length of the 

 trunk. But that is doubtless only a difference 

 of secondary importance, and may be due to 

 the great forward extension of the nipple rows 

 in the Placentals. Occasionally, however, in 



