376 



WHALES 



surface, water cannot act as a stimulus in their case, while air does, so 

 that there is no special danger to Cetaceans in being born tail first. 



Compared with other mammals, Cetaceans have a fairly thick umbilical 

 cord (Fig. 212), in which the blood vessels are twisted tightly, thus 

 increasing its rigidity (cf. Figs. 208 and 209). On the other side, the cord 

 is studded ^vith strange brown knobs, the so-called amnion pearls, and 

 though such knobs are found in various terrestrial mammals as well, their 

 function is still unknot vn. Naaktgeboren and Zwillenberg have shown 

 that they appear under the influence of some substances regulating 

 growth and differentiation of the embryonic skin. In most other mammals 

 (e.g. the cow) they disappear before birth, and the fact that this is not 

 so in whales may be correlated with a different way of cornification due 

 to the absence of hair. The part of the umbilical cord nearest the foetus's 

 abdomen is clearly thickened (Fig. 203), and covered with normal 

 cutaneous tissue. It is where this tissue gives way to ihe rest of the cord 

 that the epithelium and connective tissue rupture at birth in some species, 

 probably because the epithelium develops a number of invaginations 

 shortly before (Common Porpoise). This weakens the area in question. 

 The umbilical arteries and veins rupture just inside the umbilicus, where 

 they have a weak spot. In this way, the cord snaps when a certain strain is 

 imposed upon it, just like the cord of foals or calves, and unlike the cord 



Figure 211. Final phase in the birth of a Bottlenose Dolphin in the Marineland Aquarium, 

 Florida. {Photograph: R.J. Eastman, AFiami.) 



