DEVELOPMENT OF MAMMALS. 539 



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

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

 P'rom this tritubercular type most of the more complex forms of teeth 

 may be derived. 



Development and PlacentaHon. — The ova of placental 

 mammals are small, even those of the Whales are " no larger 

 than fem-seed." They are formed from germinal epithe- 

 lium, 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 

 mother-sperm-cell, but the spermatozoon which results from 

 the division of the mother-cell is the physiological equivalent 

 of the ovum. 



No one has succeeded in observing any extrusion of polar 

 bodies in the maturation of the mammalian ovum, but it is 

 not unlikely that analogous processes occur at an early stage. 



The ovum having burst from the ovary is immediately 

 caught by the fimbriated mouth of the fallopian tube and 

 begins to pass down the oviduct. There it is met by ascend- 

 ing spermatozoa, received by the female as the result of 

 sexual union, and is fertilised. One of the spermatozoa 

 enters the ovum, and sperm-nucleus unites with ovum- 

 nucleus in an intimate and orderly manner. It is interesting 

 to remember that it was only in 1843 that the union of 

 spermatozoon and ovum was for the first time detected by 

 Martin Barry, and in the case of the rabbit. 



