188 



sides, without necessarily interrupting breathing, and without 

 danger of choking. Incidentally, this condition persists in 

 the adult. 



Such an an arrangement is found also in all the Cetaceae 

 and in scattered members of all the other mammalian orders. 

 In the case of the marsupial mammary foetus it lends credi- 

 bility to the theory first proposed by Seiler (1828) that the 

 young does not suck, but has its milk pumped into it by a 

 muscle in the maternal marsupium which is homologous to the 

 cremaster of the male and which he called the iliomarsupialis 

 or compressor mammae. This so well developed in some 

 forms (e.g., Thalacynus see Cunningham, 1882, pi. 4) as 

 to provide an obvious mechanism for the purpose. 



On the other hand, in the opossum there seems to be no 

 mechanism for the pumping of milk, and the newborn foetus 

 can be seen and heard to suck. Indeed, it is only by suction 

 that it draws the nipple through the narrow, external aperture 

 of its mouth. It seems certain that not only the opossum but 

 all newborn marsupials can suck. The pumping of milk by 

 the mother is not universal, and is only a supplemental mecha- 

 nism if it occurs. 



The 'tubular' muscle fibers. The functional muscles in the 

 newborn opossum are histologically unique. Selenka has il- 

 lustrated them beautifully in longitudinal and cross-sectional 

 views (1887, pi. XXIX, figs. 6 and 7). The fibers are striated, 

 but the nuclei, instead of being disposed peripherally, are 

 aligned in a single axial row. All the striated fibril! ae are 

 confined to the periphery. The cytoplasm between the nuclei 

 is clear and undifferentiated. 



I know of only one other instance in which the nucleus of a 

 striated muscle cell is located in a non-fibrillar central core. 

 This is in the heart muscle of the gasteropod mollusc, Syco- 

 typus canaliculus (Dahlgren and Kepner, '08, fig. 91), but in 

 this case, as in heart muscle generally, there is only one 

 nucleus in each cell; whereas, in the opossum's tubular fibers 

 there are numerous nuclei in tandem along the non-fibrillar 

 core. 



