“RCLIPSE” PLUMAGE IN THE MALLARD. 43 
In this case the second eclipse occurred at the normal period. 
As, however, small nodules of regenerated testicular tissue 
were found at the autopsy (as indeed they were in every other 
case), we are not at present in a position to say whether a 
Mallard which is absolutely without testicular tissue will con- 
tinue to pass through the same seasonal changes of plumage as 
the normal bird. 
It is a remarkable fact that the grafts were fully spermatogenic 
in the month of September, an occurrence altogether abnormal 
in the testicle of the entire bird. We can offer no explanation of 
this anomaly. 
The delay above referred to has its parallel in the well- 
established fact that if a colt is castrated when shedding its 
winter coat, the shedding is for a time arrested, and then 
proceeds only very slowly. 
It is of interest to observe here that in the case of the Wild 
Duck, when females assume the male plumage (a phenomenon 
well known also in the Common Pheasant and other birds), the 
spurious males undergo the seasonal eclipse, although this is some- 
what incomplete and aberrant. 
IV. Removal of the testes during the eclipse does not produce 
any constant, appreciable effect upon the next passage of the bird 
into winter plumage. 
It would appear from these observations that the seasonal 
change of plumage in the Mallard is not connected with the 
spermatogenic function of the testicle. 
But whether a second function of the organ, viz. the production 
of an internal secretion, or hormone, is responsible for the change, 
could only be proved by castration so effectively carried out as to 
exclude absolutely any reproduction of testicular tissue. 
The only method of ensuring this is to reopen the abdomen 
after the castration, and remove any nodules of reproduced 
tissue. Our results in this direction we may lay before the 
Society on some future occasion. 
