THE EARLY DEVELOPMENT OF THE MAMMAL 369 



of the embryonic layers, and the formation of the embryo and 

 its chief rudiments. This will be followed by a brief account 

 of the development of external form of the human embryo. 

 Then in conclusion we shall outline the more salient facts 

 regarding the embryonic membranes and appendages, and the 

 establishment of those relations between the embryo and the 

 maternal organism which are such fundamental characteristics 

 of the true (Eutherian) Mammals. For the whole subject of 

 mammalian organogeny the student may be referred to the 

 excellent and recent accounts given in such texts as those of 

 0. Hertwig, Keibel-Mall, Minot, McMurrich, etc. 



The whole life-history of the Mammal may be roughly 

 divided into four periods, each marked by one or two striking 

 characteristics, but often not otherwise clearly separated. 

 First is the true embryonic period or period of gestation, during 

 the greater part of which the organism is retained within the 

 uterine cavity of the mother, drawing its nourishment from the 

 uterine walls. This period extends from the time of fertiliza- 

 tion to the time of birth, and its duration is widely variable in 

 different species, though usually quite constant in any single 

 form, due, perhaps, in part, to the fact that conditions of tem- 

 perature, nutrition, etc., are subject to only slight variation. To 

 mention a few examples, the period of gestation is, in the mouse 

 twenty to thirty days (Daniel), rat about twenty-one days, in 

 the rabbit thirty to thirty-two days, guinea-pig sixty-four to 

 seventy days, cat about nine weeks, dog fifty-nine to sixty- 

 three days, sheep about twenty-one weeks, pig about four 

 months, cow about nine months, man about nine months 

 (270-280 days), deer ten months, horse about eleven months, 

 elephant about twenty months. 



The time of birth, or parturition, marks the most abrupt 

 physiological and morphological transition in the entire life- 

 history. There is wide variation, among different forms, of 

 the comparative stage to which development has proceeded 

 when this event occurs. Some organisms, like the calf or the 

 colt, may be termed precocious, since they are able, within a 

 few hours after birth, to run about actively and to live with a 



