THE DEVELOPMENT OF THE EMBRYO ITSELF. 127 



least resistance through the ductus arteriosus into the aortic 

 arch beyond tlie point where its great branches emerge. It will 

 be seen that the blood going to the head and upper parts of the 

 body is greatly more valuable as nutritive pabulum than the rest, 

 especially in the quantity of oxygen it contains ; that the blood 

 of the foetus, at best, is relatively ill-supplied with its vital essen- 

 tial; and as a result we find the upper (anterior in quadrupeds) 

 parts of the foetus best developed, and a decided resemblance be- 

 tween the mammalian foetus functionally and the adult forms of 

 reptiles and kindred groups of lower vertebrates. But this con- 

 dition is well enough adapted to the general ends to be attained 

 at this period — the nourishment of structures on the way to a 

 higher path of progress. 



As embryonic maturity is being reached, preparation is made 

 for a new form of existence ; so it is found that the Eustachian 

 valve is less prominent and the foramen ovale smaller. 



PERIODS OP GESTATION. 



As a rule, the shorter the period of gestation the more nu- 

 merous the offspring at a single birth and the greater the num- 

 ber produced within the lifetime of the animal relatively to its 

 duration. Thus, on account of the number of young produced 

 by the rabbit at one birth, the short period of gestation, and the 

 frequency with which impregnation occurs, there is a much 

 larger number of progeny, short as is the animal's life usually, 

 than in the case of the cow, for example, that may bear young 

 for a much longer period. 



The following table gives approximately the duration of the 

 period of gestation of some of our domestic animals and their 

 wild allies : 



Guinea-pig (cavy) 3 weeks. 



Rabbit, squirrel, rat 4 '' 



Ferret 6 " 



Cat 8 " 



Dog, fox 9 " 



Lion 4 months. 



Sow 4 



Sheep, goat ; 5 " 



Bear 7 



Reindeer ". 8 



Cow, buffalo 10 



