DE VELOPMENT AND PL A CENT A TION. 665 



the placenta has become greatly developed, and for a prolonged period 

 serves to feed the embryo, thus reducing the importance of the milk 

 diet. In connection with the theory that the adaptation of the mouth 

 to the sucking function in Marsupials has led to the suppression of 

 the milk-teeth, it is interesting to note that conversely in Ungulates, 

 where the milk-teeth are exceedingly well developed, the milk diet, 

 according to some observers, is often relatively unimportant, the young 

 being able at a very early age to take other food. 



Wilson and Hill believe that there are vestiges of teeth in connection 

 with the permanent molars, and that these therefore belong to the 

 second set. But Leche and others believe just as strongly that the 

 molars are milk-teeth without calcined successors. 



On the many other interesting problems connected with the teeth of 

 Mammals we cannot dwell here, but it is interesting to note the 

 relation in particular cases between the diet and the form of the teeth. 

 Thus the dolphins, which feed on fish and swallow them whole, have 

 numerous almost uniform, sharp, recurved, conical teeth, well suited to 

 take a firm grasp of the slippery and struggling booty. To a slight 

 extent the same piscivorous dentition may be seen in seals. In the 

 more strictly carnivorous Mammals the incisors are small, the canines 

 are long and sharp, piercing the prey with a deathful grip, while the 

 back teeth have more or less knife-like edges, which sever flesh and 

 bone. In typical insectivorous Mammals the upper and lower incisors 

 meet precisely, "so as readily to secure small active prey, quick to 

 elude capture but powerless to resist when once seized," while the 

 crowns of the molars bear many sharp points. Herbivorous Mammals 

 have front teeth suited for cropping the herbage or gnawing parts 

 of plants, the canines are small or absent, the molars have broad 

 grinding crowns with transverse ridges. In omnivorous Mammals the 

 incisors are suited for cutting ; the canines are often formidable weapons 

 in the male sex ; the molars have crowns raised into rounded tubercles. 

 Teeth with broad rounded tubercles, as in pigs, are called bunodont ; 

 with crescentic ridges, as in sheep, selenodont ; and there are many 

 other terms. 



It is likely that the most primitive type of mammalian tooth was 

 a simple cone, such as may be seen in toothed whales. But as regards 

 back teeth there is strong evidence that the type from which all forms 

 may be derived was the tritubercular tooth, in which the crown bears 

 three cusps, disposed in a triangle. 



Development and placentation. — The ova of Mammals, 

 except Monotremes, are small ; even those of the Whales are 

 "no larger than fern seed." They are formed from germinal 

 epithelium, the cells of which grow inwards in clustered 

 masses into the connective tissue or stroma of the ovary. 

 In each cluster one cell predominates over its neighbours ; 

 it becomes an ovum ; the others invest and nourish it, and 

 are called follicle cells. 



In the middle of each clump or Graafian follicle, a cavity 



