GENERATION. 135 



the allantois is in reptiles and birds. (See Respiration, 

 p. 62.) 



The form of the placenta varies in different orders of 

 Mammalia. The shaggy coat may disappear except a com- 

 paratively small circular area forming discoidal placenta, as in 

 Bimana (man), Quadrumana (monkey), Cheiroptera (bat), In- 

 sectivora (mole), Rodentia (rat). The shaggy coat may remain 

 entire, and be at all points in contact with decidua vera, and is 

 (1) diffuse with cotyledons, as in Ruminantia (sheep), or diffuse 

 without cotyledons, as in Omnivora (hog), Cetacea (whale), Edent- 

 ata (sloth); or (2) modified in the form of a ring running 

 transversely round the fetus zonular as in Carnivora (dog). 



In implacental mammals the young, at an early period, is 

 removed from the uterus, to be attached to the nipple of a 

 mammary gland within an inguinal pouch, as in Marsupialia,* 

 or in the absence of a pouch, after a manner imperfectly un- 

 derstood, as in Monotremata. 



Mammary gland. This is a cutaneous gland belonging to 

 the racemose type, composed of aggregations of numbers 

 of simple glands, analogous to sebaceous follicles. Lacteal 

 ducts from the lobules converge to a central space to form a 

 conical spongy prominence, the nipple ('teat,' 'dug,' 'pap'). 



* It is not known in what manner the embryo is transferred from the 

 uterus to the marsupium. From observation made upon the female kanga- 

 roo by Prof. Owen at the Zoological Gardens, London, in 1830, it was shown 

 that the removal was effected during night of the thirty-ninth day of gesta- 

 tion. The following is a description by Prof. Owen of the embryo on the 

 day after its transfer : 



" The new-born kangaroo was attached to the left superior nipple, to the 

 point of which it adhered pretty firmly. It measured one inch from the 

 mouth to the root of the tail, was quite naked, and covered by a thin semi- 

 transparent vascular integument; the place of attachment of the umbilical 

 cord was obscurely indicated by a longitudinal linear cicatrix. The forelegs 

 were longer and stronger than the hind ones, and the digits were provided 

 with claws ; the toes were developed on the hind legs : the body was bent 

 forward, and the short tail tucked in between the hind legs. This little ani- 

 mal breathes strongly, but slowly : no direct act of sucking could be per- 

 ceived. Such, after a gestation of thirty-eight days, is the condition of the 

 new-born young of a species of kangaroo of which the adult, when standing 

 erect on his hind feet and tail, can reach to a height of seven feet." 



