MAMMALIA. 437 



The membranous labyrinth of the internal ear becomes more 

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

 true of the cochlea, which becomes spirally coiled. The mid- 

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

 two as -in the lower groups of vertebrates, where such connec- 

 tion exists at all. 



450. The Urinogenital Organs. As in the other vertebrates there is 

 close connection between the excretory and reproductive organs in mam- 

 mals. 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 mammals. 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 dis- 

 tinct, 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 the Anthropoidea). 



451. 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 Mono- 

 tremes there is much yolk, as among the birds. The segmen- 

 tation in the placental mammals is complete but not neces- 

 sarily equal. A solid sphere of cells is formed which becomes 

 differentiated into an outer enclosing layer (the trophoblast, 

 Fig. 234) and an inner mass of cells (Fig. 234, ent.). It is the 

 mass of cells that gives rise to the embryonic layers, from 

 which are produced the adult organs. The trophoblast^Tias 

 little or no part in the formation of the embryo proper, but 

 has a part in forming the fcetal membranes so important in 

 the group. The steps of embryonic development, while sim- 

 ilar in general to those described 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 



