5o6 CHORD AT A. 



CHAPTER XXVIII. 



MAMMALIA— (Continued.) 

 Sub-Class III.— Eutheria. 



TYPES 2 AND 3, HORSE AND OX; 4 AND 5, DOG AND CAT. 



The Eutheria mark the culminating point in mammalian 

 structure and, as might be supposed, the members of this 

 sub-class show the greatest diversity of adaptive modifica- 

 tions. As a general rule we may say that the haemal form 

 of embryonic nutrition is highly developed, the chief organ 

 forming the haemal placenta being the allantois. The yolk- 

 sac placenta, when formed at all, is merely a transitory 

 structure of little functional significance. Further advances 

 upon the metatherian type are found in the reproductive 

 organs. The urogenital and anal openings are, as a rule, 

 quite distinct, the perinaeum separating the two orifices : 

 this is especially evident in the male. The lowest part of 

 the oviducts, the vaginae, are always fused together and with 

 few exceptions the second or uterine portion shows also 

 varying grades of fusion, such as the bicornuate and 

 bipartite uterus : this fusion of the uteri probably has 

 partly to do with a reduction in the fecundity. Correlated 

 with the high development of a haemal gestation there is a 

 tendency to a reduction in the period of lacteal gestation, 

 though the mammary glands are still well developed and the 

 mammae are permanent. 



In the skeleton there are important features. The teeth 

 are typically diphyodont and heterodont, and it is usual to 

 derive the very numerous modifications from the typical 

 dentition of %W%. This typical dentition is, indeed, only 

 found in very few types, of which perhaps the pig is the best 

 known ; but the assumption of loss of certain teeth in some, 



