172 EDWARD MCCRADY, JR. 



Chondrification has at last begun in the dorsal part of the 

 periotic capsule or petrosal bone. Trachea! cartilages are 

 forming. 



The second half of the thirteenth day 



Stage 35. External distinctive features. The gestation 

 period. Parturition. The migration to the pouch. The 

 negative geotropism. The attachment to the nipple. The 

 respiratory bronchioles. The intranarial epiglottis and the 

 pumping of milk. The l tubular' muscle fibers. Testis cords 

 and the urinogenital organs. The heart and the cardiac veins. 

 The stomach and the intestines. The sensory organs. The 

 cisterna chyli. The auditory ossicles. Miscellaneous details. 



External distinctive features. Stage 35 includes specimens 

 shortly before birth (17173) and shortly after birth (17174 

 and 17175). All of these specimens are distinguished from 

 stage 34 by the almost complete resorption of the oral shield 

 (fig. 55). Of course, the specimens after birth also show some 

 external changes associated with the new mode of life, as the 

 navel in place of the umbilical cord, and the thicker trunk due 

 to the inflation of the lungs and the distension of the stomach 

 with milk. 



The gestation period. Estimates of the duration of gesta- 

 tion, ignoring for the present the uncertainty about the exact 

 time of fertilization, should be based upon the observation of 

 both copulation and parturition in a considerable number of 

 cases. Such information is not easy to obtain in the opossum. 

 The complicating factors are several. Both copulation and 

 parturition occur most often at night. Copulations which take 

 place during daylight hours seem to be most frequently sterile 

 (see chapter I). But whether fertilization occurs or not, a 

 series of changes simulating pregnancy follows each ovula- 

 tion. There is no visible enlargement of the abdomen even 

 in late pregnancy, for the newborn opossums are so small that 

 even a large litter takes up no more room that a spoonful of 

 baked beans. Any meal will change the shape of the female 



