“ BCLIPSE” PLUMAGE IN THE MALLARD. 29 
puts off the eclipse for the winter plumage. During October and 
November, when the brilliant plumage is fully declared, the testes 
increase slowly in size, although they remain of a yellow or 
brownish colowr, vetain their firmness on section, and eaude no 
fluid when incised. Spermatogenesis does not commence until 
December (perhaps the end of November) ; and the testes at first 
do not greatly increase in size. There is some variation as to the 
precise time at which spermatogenesis begins, but in any case 
the bulk of the testes greatly augments during the latter half 
of January and February. Evolution proceeds more and more 
rapidly during the second half of February, and March, until by 
the end of the latter month, or during April, the glands attain 
their maximum size. 
It must be understood that in the above account we have 
endeavoured to give an average picture of the annual evolution 
and involution of the testes, but there is no doubt that a con- 
siderable range of variation obtains in the time of the onset of 
spermatogenesis. Furthermore, in some birds, spermatozoa may 
be found in the testes at a somewhat later date than is usual; 
in such circumstances it is probable that spermatogenesis arose 
correspondingly late. 
IT, 
Observations made to study the results of Castration 
upon the Plumage. 
It was our intention to castrate a series of ducks in order to 
see whether any aberrations would result in the natural history 
of the plumage, but in spite of repeated attempts, it was found 
impossible to completely extirpate the glands. Every visible 
trace might be removed, after an extensive laparotomy, and yet, 
when the bird was killed some months later, a greater or lesser 
amount of regenerated testicular tissue was found either in the 
normal position or engrafted upon the neighbouring organs. The 
castration in every case was carried out under an anesthetic. 
The following experiments should all be read in the light of this 
fact, viz. that some regeneration of testicular tissue must have 
taken place within a few weeks or months of the operation ; and 
since we do not know how small an amount of this tissue may be 
sufficient to exert an action on the body, we cannot say whether 
the results observed within the first few months of the operation 
would have persisted had the castration been complete. There 
are, however, certain considerations which lead us to believe that 
our results, in so far as they apply to the first six or eight months 
after the removal of all visible testicular tissue, are essentially 
comparable with those which would occur during this time were 
it possible to obviate the partial regeneration of the gland. In 
the first place the delay in the assumption of the eclipse in the 
birds submitted to operation from December to April (when the 
plumage is masculine), indicates that the removal of all visible 
testicular tissue has produced a definite effect on the bird; 
