230 THE HUMAN SPECIES 



life present a general external appearance almost indistin- 

 guishable from that of any other mammal. Closer examina- 

 tion, however, reveals a series of peculiarities specifically 

 human. 



The smaller number of " primitive segments " constitutes an 

 early difference between human embryos and other mammals. 

 The jaw and the arch of the pharynx are much more delicately 

 joined in man, and the projection of the lower jaw gives an un- 

 mistakable suggestion of a lip. The brain is much further 

 developed both in the actual details of the fore-, mid- and hind- 

 brains, and in the size of the whole cranium. The three sec- 

 tions of the brain are of fairly equal size in man, while in the 

 embryo chick (to take an example) the mid-brain is dispropor- 

 tionately large. 



With the exception of the monotremes and marsupials, the 

 embryos of all other mammals, including man, are completely 

 developed in the maternal womb. 



The duration of pregnancy varies according to the body- 

 weight of the mother and the fcetus, as the following table 

 shows : 



DURATION OF PREGNANCY. 



Elephant .... 90 weeks 

 Giraffe ..... 63 ,, 



Mare 48 



Cow ..... 40 



Sheep, goat .... 20-21 ,, 



Pig 17-18 



Bitch 8-9 



Cat 7-8 



Rabbit 5 



In man the duration of pregnancy is very long in proportion 

 to the body-weight. The time is about forty weeks, the same 

 as in cattle, but the sex of the fcetus makes a slight difference, 

 for the female develops rather more quickly and on that account 

 is born a little sooner. 



The numerical superiority of the male sex exists as a rule 

 among domestic animals just as in man/ 



The percentage of males to females is as follows : 



