92 J. BEARD, 



mammae, or, seeing that the two are in close association in marsupials, 

 of the mammary apparatus. 



In various contributions Klaatsch^) has given us a good deal 

 of insight into the relationships of three important structures, the 

 mammae, the mammary pockets, and the marsupium. 



But, although from the side of comparative anatomy he has eli- 

 cited much of their history, so far the development in any Metatheria 

 has not been recorded. The series of stages afforded by the three 

 specimens of the present paper does not extend to a period of the 

 development sufficiently late to yield a complete picture of the de- 

 velopment of the mammary apparatus, none the less, the oldest stage 

 of the three (M. (hetidis) is so far advanced, that it is not difficult 

 to gather, of what nature the subsequent changes must be, in order 

 that a pouch, containing mammary glands, may be formed. 



As Oscar ScHULTZE'has demonstrated, in many Eutherian mammals 

 the development of the mammary apparatus is initiated by the form- 

 ation, on each side of the body, of an epiblastic thickening, the mam- 

 mary line ^). This structure is met with in stages, somewhat pre-critical, 

 in rabbit, mole, pig, etc. and possibly in man (Hugo Schmidt) and, 

 whilst present in the pig, it is absent in other Ungulata (Schultze). 



No mammary line, properly speaking, is, so it would appear, laid 

 down in Trichosurus, which possesses only two mammae, and the 

 formation of the apparatus begins somewhat before the critical period 

 in the shape of an inward proliferation of the Malphigian layer on 

 each side of the groin-region and beneath the thick epitrichium (Fig. 4). 



In the newly-born pouch- foetus the proliferation has resulted in 

 a somewhat conical eminence on each side, filled with cells of the 

 Malphigian layer (Fig. 5). 



Macropus thetidis , of which the third specimen is a represen- 

 tative, possesses in the adult condition two pairs of mammae , and 

 this increased number naturally gives rise to other conditions in the 

 development. 



In the specimen, a female, the apparatus is represented by two 



1) Klaatsch, H., Zur Morphologie der Säugethierzitzen, in : Morph. 

 Jahrb., V. 9, 1883. — Ueber die Beziehungen zwischen Mammartascbe 

 und Marsupium, in: Morph. Jahrb., V. 17, 1892. — Ueber Marsupial- 

 rudimente bei Placentaliern, in: Morph. Jahrb., V. 20, 1893, p. 276—288. 

 — Studien zur Geschichte der Mammarorgane , in : Semon's Zool. 

 Forschungsreise, V. 2, Lief. 2, 1895. 



2) Identified by Klaatsch as a "marsupial ridge". 



