REPRODUCTION 361 



in Other animals, and whale biologists are therefore in a specially favour- 

 able position. Unfortunately, the corpora albicantia give no indication 

 whether pregnancy followed a given ovulation. True, some biologists 

 (e.g. Zemski, 1957) claim that from the size, shape and structure of 

 the corpora, one ought to be able to tell whether a given white body was 

 associated with pregnancy or not, but so far the evidence has remained 

 inconclusive. However, research into this problem is being continued and 

 it may well be possible one day to tell ho\v many calves a given whale has 

 given birth to. 



Whenever ovulation goes hand in hand ^vith mating, the ovum liberated 

 from the follicle is likely to meet spermatozoa in the upper oviduct. Some 

 of these will penetrate the membrane of the egg cell, and when one fuses 

 with the cell itself, the ovum is fertilized and is forced into the uterus by 

 contractions of the muscular walls of the oviduct. During its journey, 

 \vhich may take a few days, the ovum develops into a tiny vesicle. At the 

 same time, the lining of the uterus is being prepared to receive it, so that, 

 on arrival, it can become attached to the uterine wall. 



In all Odontocetes so far investigated, the fertilized ovum Avas almost 

 invariably attached to the distended left horn of the uterus, while the 

 smaller right horn was found to contain a part of the allantois (Figs. 201 

 and 209). Sleptsov (1940) states that in only 17 per cent of the 635 

 pregnant dolphins and Belugas which he investigated was the embryo 

 found in the right horn. In Mysticetes, on the other hand, the foetus may 

 develop in either horn, though it appears from investigations of Blue and 

 Fin Whales that there is a slight balance (60 to 65 per cent) in favovu- of 

 both the right ovary and the right horn. 



Like that of man, apes, horses and cows, the Cetacean ovary usually 

 produces no more than one ovum at one time. Occasionally, however, 

 more than one ovum may be discharged, when twins or multiplets may 

 develop. In whales, we know a great deal about this subject, since the 

 uterus of every captured female is investigated carefully for the presence 

 of embryos. If the embryos are very small, they may well escape the 

 watchful eyes of the inspectors, but larger foetuses rarely do. From 

 whaling statistics it appears that the percentage of twins conceived by 

 Blue, Fin, Sei and Humpback Whales ai-e 0-68, 0-93, i -09, and 0-39 of 

 total pregnancies respectively. These figures correspond by and large 

 with those found in man (i -3 per cent), horses (i -i per cent) and cows 

 (0-5 to I '9 per cent). According to Kimura (1957) about 33^ per cent 

 of Fin Whale twins are uniovular. Multiplets seem to be more common 

 in \vhales than in man and in larger domestic animals, for triplets, and 

 to a lesser extent even quadruplets and sextuplets, have all been described 

 in the literature. It is, of course, impossible to say whether the entire 



