REPRODUCTION 37I 



unnoticed by the mother. These and the later contractions are called 

 peristaltic contractions, i.e. wave-like contractions throughout the entire 

 uterus, starting at the apex of the horn and continuing right up to the 

 cervix. It would appear that these peristaltic contractions have a tendency 

 to pull the lightest and least rigid parts of the foetus with them and thus 

 to bring them nearer the cervix and the genital slit. With the onset of 

 labour pains shortly before birth, the neck and the fore-legs are stretched 

 out, so that the head is born resting on these limbs. 



There is, however, one serious danger associated with peristaltic con- 

 tractions, viz. the danger of compressing the umbilical cord which joins the 

 foetus to the placenta and consists of blood vessels. Since the cord is very 

 light and mobile (Figs. 204, 208, and 212), there is a tendency for it to be 

 driven towards the cervix, just like the head, with the consequent risk of a 

 fatal compression when the head, in passing through the narrow cervix, 

 presses against it. Alternatively, the umbilical cord may wind round the 

 throat of the foetus, and cut off the blood supply to its head. To minimize 

 these risks, the umbilical cords of all the animals in question are exception- 

 ally short, i.e. they represent from 20-60 per cent the total length of the 

 foetus, unlike the umbilical cord of man and other primates (which have 

 no peristaltic contractions of the uterus) where the corresponding figures 

 range from 100-200 per cent. 



As far as Cetaceans are concerned, we do not know whether the con- 

 tractions of the uterus are, in fact, peristaltic, but we do know that the 

 structure of their uterine walls and particularly the arrangement of 

 longitudinal and annular muscles in them are identical with the structure 

 of the uterus of those mammals in which peristaltic movements are known 

 to occur, and unlike the uterine structure of primates in which the uterus 

 simplex contracts as a whole. It seems likely, therefore, that peristaltic 

 movements occur in the Cetacean uterus as well, the more so since the 

 umbilical cord represents about 40 per cent of the foetus's average length 

 at birth. Moreover, conditions in the abdominal cavity are almost 

 identical with those found in Ungulates. True, the genital slit is not as far 

 above the uterus as it is in Ungulates, but Fig. 207 shows clearly that the 

 passage from the uterus to the vagina (the cervix) is dorsally placed and 

 that the uterus itself occupies a lower position. 



For all these reasons, we might have expected Cetacean calves to be 

 born head first, were it not for the fact that the shape of the Cetacean foetus 

 differs so characteristically from that of terrestrial mammals. In Cetacean 

 foetuses, the head and thorax are the most bulky and also the most rigid 

 parts, as they have hardly any neck to speak of and as their hindquarters 

 and long tail are light and very mobile (Fig. 210). For this reason, the 

 head and neck are forced to the lower front of the uterus, while the tail 



