IV. VEHTEBRATA. 553 



Amphibia. Urinary and sexual ducts then either open into the 

 urogenital sinus, the lowest part of the bladder leading to the 

 cloaca (turtles, inonotremes), or this part receives only the geni- 

 tal ducts, while the ureters enter the base of the bladder. The 

 urogenital sinus remains in connexion with the cloaca in the 

 turtles and monotremes; in the other mammals a cloaca occurs 

 only in embryonic life. Later, by formation of the perineum, the 

 cloaca is divided into a hinder digestive and an anterior urogenital 

 canal. Step by step the stages may be followed from urogenital 

 ducts opening behind to those opening in front of the anus. 



Asexual and parthenogenetic reproduction are unknown in the 

 vertebrates. The impregnation of the eggs in the lower groups 

 is usually external and occurs during oviposition; in the higher 

 internal copulation is effected by opposition of the genital ori- 

 fices or by the development of an intromittent organ, the penis. 

 The fertilized egg can undergo a part or the whole of its devel- 

 opment in specialized parts of the oviduct (uterus). Accordingly 

 viviparous and oviparous forms are distinguished, and between 

 these extremes those that are ovo viviparous (cf. p. 161). Most 

 elasmobranchs are viviparous, but many are oviparous. In the 

 teleosts oviparous forms predominate, but there are viviparous 

 exceptions. So, too, among the reptiles and Amphibia there are 

 some viviparous species among the egg-laying majority. The 

 birds and mammals are most constant, the first being exclusively 

 ovoviviparous, while all the mammals bring forth living young 

 with the exception of the ovoviviparous monotremes. 



Three embryonal appendages may occur in the development, 

 the yolk sac, the amnion, and the allantois. The yolk sac is small 

 in those vertebrates which have some yolk, but not enough to 

 cause meroblastic segmentation (Amphibia), yet it is everywhere 

 present and is best developed in those groups (fishes, fig. 582, 

 reptiles and birds) with discoidal segmentation, and is the result 

 of the accumulation of food material in the digestive tract, which 

 forces out its ventral wall like a hernia. Its presence in the mam- 

 mals, which have small eggs lacking in yolk, is an indication that 

 these have descended from large-yolked forms, such as the mono- 

 tremes yet are. The embryo either lies directly on the yolk or is 

 connected with it by a yolk stalk. 



While the yolk sac is widely distributed, the amnion and allan- 

 tois are restricted to reptiles, birds, and mammals, which are con- 

 sequently spoken of as Amniota or Allantoidea, in contrast to the 

 fishes and Amphibia, which are frequently called Anamnia or Anal- 



