432 THE REPRODUCTIVE FUNCTIONS. 



views of the future circumstances and wants of the system. 

 The double origin of all the parts of the frame, even those 

 which appear as single organs, and the order of their forma- 

 tion, wliich, in each system, commences with the parts most 

 remote from the centre, and proceeds inwards, or towards 

 the mesial plane, are among the most singular and unex- 

 pected results of this train of inquiries.* We cannot but be 

 forcibly struck with the numerous forms of transition through 

 which every organ has to pass before arriving at its ultimate 

 and comparatively permanent condition: we cannot but won- 

 der at the vast apparatus which is provided and put in action 

 for eflecting all these changes; nor can we overlook the in- 

 stances of express contrivance in the formation of so many 

 temporary structures, which are set up, like the scaffold of 

 an edifice, in order to afford the means of transporting the 

 materials of the building in proportion as they are wanted; 

 nor refuse to recognise the evidence of provident design in 

 the regular order in which the w^ork proceeds, every organ 

 growing at its appointed time, by the addition of fresh par- 

 ticles brought to it by the arteries, while others are carried 

 away by the absorbents, and gradually acquiring the form 

 which is to qualify it for the performance of its proper ofTice 

 in this vast and complicated system. 



• The first of these two laws is termed by Serres, who has zealously pro- 

 secuted these investigations, " la hi dc symmHrit;'" and the second, "^f hi 

 de conjugaison.*' He maintains that they are strictly applicable to all the 

 parts of the body having- a tubular form, such as the trachea, the Eustachian 

 tube, the canals, and perforations of bones, &,c. See the preliminary dis- 

 course to his *' Anatomie comparee du cerveau," p. 25; and also his several 

 memoirs in the " Annales des Sciences Naturelles," vols. xi. xii. xvi. and xxu 



An excellent summary of the principal facts relating to the development 

 of the embryo is given by Mr. Herbert Mayo, in the third edition of his 

 ^* Outlines of Human Physiology.'* 



