THE EMBRYOLOGY OF THE OPOSSUM 



not anaesthetized. My assistant laid her on her back on the operating 

 table, held the back of her neck with his left hand, and held her pouch 

 open with his right. This is the simplest procedure, and the most 

 desirable when possible; but if the mother becomes too excited, as 

 is sometimes the case, she must be anaesthetized. We have usually used 

 an ether cone for this purpose. If any of the ether passes through 

 the mammary gland into the milk, it is not enough to have any 

 recognizable effect on the young in the pouch. 



In the removal of the lens from the eye the epitrichium between 

 the lids was cut with a small pair of spring scissors the tips of which 

 had been specially ground to a very fine point. Then a small sliver 

 of steel broken from the edge of a thin safety razor blade, and held 

 in a small needle chuck, was used as a knife for slitting the anlage of 

 the cornea. When this was cut, gentle pressure on the side of the 

 optic cup with the tip of a dull probe, caused the lens vesicle to pop 

 out. If the original incision through the epitricium is along the line 

 of fusion of the lids, there is no bleeding in this operation. The 

 incisions were allowed to close by themselves. 



In the amputation of the legs three methods were tried by Dr. Olof 

 Larsell and myself, and only one method found satisfactory. Cauteri- 

 zation and simple snipping off with fine scissors both lead to high 

 mortality. On the other hand, if a loop of fine, white, cotton thread 

 is tied tightly about the limb, the portion distal to the ligature can 

 be cut off immediately or can be left until the following day and then 

 cut off. Survival in this case is usually excellent, and not even a 

 scar can be found on the stump at later stages. Occasionally the 

 ligature slips, or is not made tight enough to begin with, allowing 

 blood to get into the limb. In several such cases the result was 

 hemostasis (due apparently to occlusion of the veins, which are super- 

 ficial, but not the arteries, which are deep), with great swelling and 

 discoloration of the limb and eventual death of the individual so 

 affected. Both hind limbs can be amputated at one time without 

 danger, but when a hindlimb and forelimb were removed from the 

 same individual it was found advisable to amputate one at a time, 

 with an interval of 24 hours for recovery after the first operation. 



Though regeneration was not the subject of these studies, it may 

 be of interest to note that none occurred in any of these cases. At 

 the time of the operation the hindlimb was only an embryonic paddle 

 with small digital buds, and the opossum is more primitive than any 

 mammal the embryo of which has hitherto been tested for regeneration. 



Both of these operations were done with the young still attached to 

 the mammary nipple. In fact, this attachment was found to be a 

 convenient mechanism for immobilization of the head. The rest of the 

 body was immobilized for the eye operation by means of a strip of 

 adhesive tape fastening the young to the mother's abdominal wall. 



