90 



INTRODUCTION TO CLASSIFICATION. 



The urinary bladder opens into a distinct urethra, which, 

 directly or indirectly, receives the vasa deferentia in the male. 



The corpus callosum is very variable in its development, 

 commonly attaining a much larger size than in the preceding 

 groups ; the optic lobes are divided into four portions. 



The young are nourished within the uterus until such time 

 as they are competent to suck milk from the teats of the parent, 

 to which end the chorion always develops processes or villi, 

 which are well supplied with vessels brought to them by the 

 allantois. These processes becoming interlaced more or less 

 closely with corresponding vascular developments of the wall of 

 the uterus (and so forming a " placenta "), an interchange of 

 constituents takes place between the foetal and the maternal 

 blood, through the separating walls of the foetal and maternal 

 vessels. In this manner, throughout its prolonged intra-uterine 

 life, the Monodelphian fostus is supplied with nourishment and 

 gets rid of its effete products. 



It is a well-established fact that two very distinct types of 



Fig. 40. 



Fig. 40. Diagrammatic section of a human pregnant uterus, with the contained ovum 

 (Longet). u, uterus ; I, oviduct ; c, cervix uteri ; du, decidua uteri ; dr, decidua 

 reflexa ; ds, decidua serotina ; ch, chorion ; am, amnion ; al, allantois ; nb, umbilical 

 vesicle ; z, villi which form the fo3tal part of the placenta ; z', villi over the rest of 

 the chorion, which take no part in the placental function in man. 



