WHALEBONE WHALES—MACKINTOSH 251 
summer Zenkovic (1937) found that they were feeding on bottom- 
living amphipods. It is not quite clear, however, whether these really 
form the staple food of the main stocks of this species. 
It is evident that the food of whales in the Northern Hemisphere 
requires further investigation. 
IV. BREEDING, GROWTH, AND AGB 
(1) THE REPRODUCTIVE ORGANS 
For the purposes of the present article it is not necessary to give a 
description of the reproductive organs (for this see Mackintosh and 
Wheeler, 1929; and Ommanney, 1932), but we have to consider 
certain changes which take place in the course of the sexual cycle, 
and especially the formation and persistence of the corpora lutea of 
the ovaries. From this point of view blue and fin whales are the best 
known, and the following particulars apply equally to both these 
species except where otherwise stated. 
The time of the breeding season has not been determined by direct 
observation, but evidence is provided by the sizes of foetuses and by 
the state of the reproductive organs. In the male a seasonal change 
in the condition of the testis has been observed. The formation of 
spermatozoa can be seen at all times of the year, but in the early 
winter there is a greatly increased proliferation of germ cells in the 
tubules. It is difficult to obtain adequate material from adult whales 
after the close of the Antarctic summer whaling season, but this period 
of activity probably lasts from about April to June or July in blue and 
fin whales in the Southern Hemisphere (Mackintosh and Wheeler, 
1929). Although the size of the testis varies considerably in different 
individuals there is no evidence of an increase in size at the breeding 
season such as Meek (1918) found in the porpoise. In the female 
information is obtained principally from examination of the uterus to 
determine whether a foetus is present or absent, the size and sex of 
foetuses, the condition of the mammary glands, and the condition of 
the ovaries, especially of the corpora lutea. In whales the corpus 
luteum (formed by proliferation of tissue in the Graafian follicle after 
ovulation) is a conspicuous body which may measure 10 or more 
centimeters in diameter. At the end of the period of gestation, or 
presumably much sooner if impregnation has not taken place, it 
shrinks to a smaller, tougher body which persists certainly for many 
years and probably throughout the life of the whale (see Wheeler, 
1930, and others). These old corpora lutea (strictly corpora albican- 
tia) are thus cumulative. The number to be found in one pair of 
ovaries varies from 1 to over 50, and, since they give an indication of 
the number of ovulations which have taken place, they give at least 
a clue to the age of the whale. 
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