The Birth-period of Trichosurus vulpecnla. 91 



The specimen is depicted three times its natural size ^) in Fig. 3. 

 Its greatest length is 20 mm 2), and the "Nacken-Länge" about 18 mm. 



Externally not very much is visible. The head is raised, the 

 mouth open, the lips fused at the sides, the fore-limbs flexed, the 

 digits of the hind-limbs are well-marked and characteristic, and the 

 tail is large. The beast is, in fact, quite like a kangaroo. 



The eye-lids are closed, and the external ears still lie under the 

 epit richium. 



No hair-follicles are visible externally. 



Internally, in section, the tongue is beset with papillae. It 

 is deeply hollowed-out above for the reception of the teat. Tooth- 

 germs are differentiating. Bone-formation has set in to some con- 

 siderable extent in both upper and lower jaws, and a bony palate is 

 in course of formation. The hypophysis exhibits much budding. 



There are very pronounced hair- follicles on the snout. Salivary 

 glands are well-developed. An intra-narial epiglottis is, of course, present. 



There is much pigment in the retina, and the latter is in course 

 of histological differentiation. The lens-cavity is almost obsolete. 



There is cartilaginous auditory capsule, and the external auditory 

 meatus is not yet open. 



In the cervical region the formation of the posterior fissure or 

 septum of the spinal cord has appreciably advanced, perhaps for Vs *^ 

 of its length (Fig. 8), in the posterior end of the thoracic region for 

 rather more, about Ve *^ (Fig- '^)- 



There is a well-ossified clavicle, and ossification has commenced 

 in the humerus. 



The blood-corpuscles are mainly nucleated. 



The mesonephros is still a prominent structure, while the per- 

 manent kidney is small. The specimen is a female. There is a mark- 

 ed formation of two pairs of mammary pockets (Fig. 6). 



The pelvis and femur are cartilaginous, and there are well de- 

 veloped epipubic cartilages. The head and thorax still greatly exceed 

 the abdomen. In other points, so far as I can see, the animal pre- 

 sents no appreciable advance on the newly-born Trichosurus. 



The Deyelopment of the Mammary Apparatus. 



The three specimens described in the preceding pages present us 

 with three progressive stages in the formation of the pouch and 



1) Preserved size. 



2) Mr. J. P. Hill has a specimen of M. thetidis in the act of 

 being born. It measures nearly 18 mm G. L. 



