The Birth-period of Trichosurus vulpecula. 93 



pairs of depressions, mammary pockets, lying beneath the epitrichium, 

 and filled with cells. 



In Fig. 6 one pair of these is depicted, and the other, lying 

 immediately in front, in the groin-region, is quite similar. At the 

 base of each depression there is nothing else comparable in appearance 

 to a "milk-point" of, say, a critical or post-critical pig. 



In such a form as M. thetidis it appears undoubted, that the 

 development proceeds very much after the fashion supposed by 

 Klaatsch, the first indication being in the form of two subepitrichial 

 thickenings of epiblast on each side, comparable to the milk points 

 of ScHULTZE, and termed by Klaatsch mammary pockets. 



At the base of each of these at a later stage there arises, in all 

 probability, a mamma, and the confluence of these mammary pockets 

 yields, as Klaatsch has insisted, the pouch or marsupium. The latter 

 author has also shown, that the milk-points of Schultze, projecting 

 at first from the surface, afterwards sink in and form mammary 

 pockets. As, strictly speaking, these in Eutheria never get beyond the 

 condition of mammary pockets, as they never come together to form 

 a common receptacle, the marsupium or pouch of Metatheria never 

 arises in Eutheria, and the mammary apparatus of the latter is arrested 

 in development, and never reaches the stage attained in most mar- 

 supials. 



As I have elsewhere demonstrated ^), the pouch of the Metatherian 

 ancestry was lost in Eutheria, if it ever existed, by the adoption of a 

 gestation of two critical units, as an advance on that of one unit 

 characteristic of almost all Metatheria. It is now of interest to notice, 

 that this loss was brought about simply by its non-formation with the 

 persistence of the mammary apparatus on the level, which immediately 

 precedes its formation in marsupials. 



This conclusion is not at all in harmony with the views of 

 Klaatsch (in: Morph. Jahrb., V. 20, pp. 276—288), who regards 

 Schultze's mammary line as the representative of a rudimentary 

 pouch. He, therefore, terms it the "marsupial ridge" (p. 286). 



The incorrectness of his conclusion is proved 1) by the fact, that 

 the mammary line, or "marsupial ridge", arises in development before 

 the milk- points, or mammary pockets, and 2) by the circumstance, 

 that the latter do not arise, as indicated in his text- figure 2 b (p. 287), 



1) Beard, J., The Span of Gestation and the Cause of Birth, Jena, 

 Gustav ï'ischer, 1897, p. 47. 



