108 COMPARATIVE ANATOMY. 



(Surinam catfish), or Hylodes lineatus (Surinam frog). En- 

 twined in a figure-of-eight coil around the hind legs, as in 

 Alytes obstetricanS) they may be contained within a large cuta- 

 neous dorsal fold opening posteriorly at coccyx, as in Osteo- 

 cephalus. Among fishes they may be held in a pouch near 

 anus, as in the male of Hippocampus (sea-horse), or in the 

 mouth of the male of Bagrus. 



The oviduct may retain the egg during a portion of the 

 time required for development, as in Chelonia, implacental 

 mammals, or during the whole of it, as in Paludina vivipara, 

 Anableps, Ditrema argenteum, Blennius (blenny), the innocent 

 serpents, and placental mammals. Such animals are called 

 for convenience viviparous. 



The placenta is a structure peculiar to placental mammals 

 (see classification, and p. 134). Placenta-like structures seen 

 in some nematoid Entozoa; Salpa; Squalus (shark), and an 

 osseous fish, Ditrema argentewn. In implacental mammals the 

 embryo is discharged at an early stage of development to be 

 attached to mammary gland within an inguinal pouch, as in 

 Marsupialia, or, in absence of pouch, after a manner imper- 

 fectly understood, as in Monotremata. 



The act of an egg at maturity escaping from the ovary is 

 termed ovulatlon. The time of its occurrence varies in dif- 

 ferent animals. In a majority of species the act is repeated 

 for a number of years consecutively, generally in the spring, 

 in terrestrial animals, and frequently several times a year; 

 most of the lower animals, however, lay eggs in the fall, or 

 during winter. In others, on the contrary, it occurs but once 

 during life, at the period of maturity, and the animal soon 

 afterward dies. (Agassiz and Gould.) Ovulation is frequently 

 accompanied with sexual excitement (see sexual characters, 

 p. 102). In Mammalia it induces a determination of blood 

 to the genital organs of the female, accompanied by sexual 

 desire. This is termed 'rut' in the deer, boar, etc., and 

 'heat' with carnivores.* Phenomena of menstruation con- 

 fined to Quadrumana and the human species. 



* The heat belongs more properly to the female than to the male, as there 

 are many species whose females receive the male only at particular seasons, 



