342 CHESTER H. HEUSER 



Hartman had collected opossum embryos for several years, and 

 as he had worked out methods for recognizing the various stages 

 of pregnancy, we were able to secure a very complete series. 



The embryos were removed from the excised uterus in Ringer's 

 solution, as described by Hartman ('19).^ With scissors a long 

 incision was first made along the uterine horn, cutting rapidly 

 through the muscular layer but without perforating the mucosa, 

 and the latter was then pulled apart with forceps. In order to 

 avoid injury to the embryonic vesicles, which during the last 

 two or three days before birth are closely pressed against each 

 other and intimately applied to the uterine mucosa, the vesicles 

 were separated from onf another by cutting off the uterine muscle 

 before opening the lumen ; even small pieces of the muscle remain- 

 ing caused considerable interference by the formation of pockets 

 in the mucosa which surrounded and held portions of the embry- 

 onic membranes. This care in separating the vesicles was taken 

 for the reason that I wished to make injections of the blood- 

 vessels in one or more embrA^os of a litter. Such injections could 

 be made very satisfactorily in these embryos with an intact 

 vascular system and a vigorously beating heart. 



The embryos which were to be sectioned were dropped while 

 alive into Bouin's fluid and left for six to twenty-four hours, 

 depending upon the size of the individual. The Bouin's fluid 

 was replaced with 80 per cent alcohol by adding the latter in 

 small quantities to the fixing fluid — taking several hours for the 

 transfer. The alcohol was later changed several times, and 

 finally lithium carbonate — a few drops of a saturated aqueous 

 solution to the ounce of alcohol — was added to remove the 

 picric acid. As the pouch-young were firmly attached to teats 

 in the pouch, they were removed by exerting a slow continuous 

 pull with blunt forceps pressed against the mouth. The speci- 

 mens were then placed at once into Bouin's fluid, and after the 

 muscular contractions had ceased, incisions were made through 

 the abdominal wall to insure the rapid entrance of the fixing 

 fluid into the body cavity. Embryos and pouch-young were 

 also fixed and stored in 10 per cent formalin. 



