REPRODUCTION 369 



but also in some Mysticetes including a Humpback Whale (Dunstan, 

 1957). The question, therefore, arises why Cetaceans differ so radically 

 from terrestrial mammals in this respect. The answer can be found by 

 following up the various suggestions of Prof, de Snoo, formerly Professor of 

 Obstetrics at the University of Utrecht, who was struck by the fact that 

 uniparous animals, whose offspring are always relatively large, usually 

 produce their young head first, while multiparous animals whose offspring 

 is much smaller usually produce 50 per cent of their young tail first, and 

 50 per cent head first. Now, if the offspring is small, it can usually slip 

 through the mother's pelvis fairly easily, and it is born fairly quickly. In 

 uniparous animals, on the other hand, whose young are born large, the 

 process of birth lasts much longer, and here it may be a question of life 

 or death whether the young emerge head or tail first. While we do not 

 know precisely ^vhat produces the first respiratory stimulus, it seems 

 probable that, in addition to cessation of the umbilical blood stream, the 

 dryness and low temperature of the air outside the mother's body play 

 an important part. Moreover, the danger of rupture of, or strangulation 

 by, the umbilical cord is much greater in caudal than in head presentation. 

 Thus the first breath might be taken when, during a caudal presentation, 

 part of the body has emerged and the head has not, with a consequent 

 intake of blood, mucous, and amniotic fluid. The danger of choking or of 

 becoming infected by what are partly non-sterile fluids is obviated by head 

 presentations in which the nostrils always emerge first, so that every 

 veterinary surgeon who can turn a caudal presentation round before or 

 during birth will invariably do so. 



To find out how the final position of the foetus is assured, we shall look 

 at mammals with a two-horned uterus, and ignore those with a uterus 

 simplex (i.e. the primates) since the discussion would otherwise take us too 

 far afield. Now, in mammals with a uterus bicornis (e.g. horses, cattle, 

 and deer), the embryo has a fairly small head, a long and very mobile 

 neck, and relatively heavy hind-quarters. Fig. 206 shows clearly that most 

 of the uterus with the major part of the foetus occupies the lower front of 

 the abdomen, while the passage through which the foetus is eventually 

 expelled is higher up. Because the space inside the abdomen is restricted, 

 because only its lower front can be distended, and probably also because 

 of the distribution of the embryo's mass and the gravitational effects on it, 

 the heavy rump of the foetus drops to the lower part of the uterus and the 

 abdomen, while the head naturally points to the cervix and the pelvis. 



This position is also ensured by the fact that the wall of the uterus keeps 

 contracting at regular intervals throughout pregnancy and particularly 

 during birth, when the contractions become intense and are felt as labour 

 pains. The original contractions are much gentler and probably go 



