MAMMALIA 447 



The organs of special sense are similar to those in the birds. 

 The ear becomes more complicated. There is usually a well- 

 developed external ear, or pinna, in the terrestrial forms, which 

 is often movable and serves to gather the sound waves. The 

 membranous labyrinth of the internal ear becomes more com- 

 plicated than in any of the lower forms. This is especially 

 true of the cochlea, which becomes spirally coiled. The middle 

 ear is bridged by a series of three bones, instead of one or two 

 as in the lower groups of vertebrates where such connection 

 exists at all. 



461. The Urinogenital Organs. As in the other vertebrates there is close 

 connection between the excretory and reproductive organs in mammals. The bean- 

 shaped kidneys communicate by ureters with a median urinary bladder, which in 

 turn has the urethra leading to the outside. The urethra also serves as the outlet 

 for the sperm in the male. The testes, which in other vertebrates lie in the body 

 cavity, pass backward and descend into a fold of the skin, in the majority of mam- 

 mals. In the female, the ovaries are in the abdominal cavity, and when the ova 

 are ripe they break forth into the cavity and pass into the fringed, funnel-shaped 

 mouth of one of the two oviducts. The oviducts may be completely distinct, 

 opening separately into the vagina (as in most rodents), in which case each has a 

 special portion in which the young are retained during early development (uterus') ; 

 or there may be found various degrees of union of the uterine portions until there 

 is a single uterus into which the two oviducts empty (as in man). 



462. Reproduction and Development. All the mammals 

 except the monotremes are viviparous. Impregnation may take 

 place in the oviduct or in the uterus. In the Placentalia the 

 ova are small and have little yolk, whereas in the Monotremes 

 there is much yolk, as among the birds. The segmentation in 

 the placental mammals is complete but not necessarily equaL 

 A solid sphere of cells is formed which becomes differentiated 

 into an outer enclosing layer (thetrophoblast, Fig. 237) and an inner 

 mass of cells (Fig. 237, ent.). It is the mass of cells that 

 gives rise to the embryonic layers, from which are produced 

 the adult organs. The trophoblast has little or no part in 

 the formation of the embryo proper, but has a part in forming 

 the foetal membranes so important in the group. The steps of 

 embryonic development, while similar in general to those de- 

 scribed for the other Vertebrata, are modified by the absence of the 

 yolk and the retention of the developing egg in the body of the 

 parent. The embryonic membranes amnion and allantois 



