THE EMBRYOLOGY OF THE OPOSSUM 



83 



Toward the end of the ninth day the vesicles, which are still 

 spherical and roll or float freely in the uterus (fig. 27, C), 

 have attained a diameter of 9 or 10 mm. The embryo itself, 

 however, is not more than about 6 mm. in greatest length, and 

 it will not exceed this length until the end of the eleventh day. 

 For between the ninth and the eleventh days several flexures 

 occur, and the 'length' of the embryo undergoes such reduc- 

 tion and fluctuation that linear dimension loses all significance 

 as a criterion of developmental stage. An embryo of 5.5 mm. 

 greatest length may belong to stage 25, or to any of the 



Fig. 27 Photographs of external views of stage 26. A, dorsal view of 16146. 

 B, ventral view of 16145 after removal of non-vascular chorion. C, vesicles and 

 part of uterine mucosa from Ilartmun's litter 291. 



stages of the tenth day (27 to 29), or to the middle or late 

 parts of the eleventh day (stage 31). After stage 31 crown- 

 rump length becomes a useful datum for identification, and re- 

 mains so until several weeks after birth when the head begins 

 to assume a more axial alignment and it is necessary to shift 

 to snout-anus measurements. 



Cephalic flexure. The first of the flexures referred to is 

 the so-called cephalic flexure, which is a right angle, ventral 

 bend in the central nervous system at the level of the mesen- 

 cephalon. It would be more proper to call this the mesen- 

 caphalic flexure, as the pontine flexure, which occurs in stage 



