DEVELOPMENT AND PLACENTATION. 647 



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. 



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

 simple cone, such as may be seen in toothed whales. In some of the 

 extinct mammals, e.g.^ Triconodon, the tooth is a main cone with two 

 lateral cusps, and this type leads to what is called the tritubercular 

 tooth, in which the crown bears three cusps disposed in a triangle. 

 From this tritubercular type most of the more complex forms of teeth 

 may be derived. But it remains doubtful whether the tritubercular type 

 is the result of the fusion of three cones, or the budding of one. 



Development and Placentation. 



The ova of placental mammals 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 

 is formed containing fluid, and into this cavity the follicle 

 cells immediately surrounding the ovum project as what is 

 called the discus proligerus. 



When mature the ovum protrudes on the surface of the 

 ovary, and is liberated by the bursting of the Graafian 

 follicle. Some blood, which fills up the empty follicle, 

 degenerates into what is called the corpus luteum. 



The spermatozoa are formed from germinal epithelium in 

 the testes. The primitive male cells or spermatogonia give 

 rise by division to daughter cells or spermatocytes, which 

 with or without further division form spermatozoa. 



The homologue of the ovum is the spermatogonium or 



