80 ADAM SEDGWICK. 



is the mode of formation of the head and trunk in these 

 animals." 



This, I think, shows quite clearly Darwin's view of the 

 matter. 



Huxley, in his ' Man's Place in Nature,' says : — " The his- 

 tory of the development of any other Vertebrate animal — lizard, 

 snake, frog, or fish — tells the same story. There is always, to 

 begin with, an egg having the same essential structure as that 

 of the dog; the yolk of that egg undergoes division or seg- 

 mentation, as it is called, the ultimate products of that seg- 

 mentation constitute the building materials for the body of the 

 young animal ; and this is built up round a primitive groove, 

 in the floor of which a notochord is developed. Furthermore, 

 there is a period in which the young of all these animals 

 resemble one another, not merely in outward form, but in all 

 essentials of structure, so closely, that the difl'erences between 

 them are inconsiderable, while in their subsequent course they 

 diverge more and more widely from one another," 



The Significance or Ancestral Rudiments in 

 Embryonic Development. 



The existence of a phase at the beginning of life during 

 which a young animal acquires its equipment by a process of 

 growth of the germ, is of course intelligible enough. We see 

 such a phase in the formation of buds, and in the sexual repro- 

 duction of both animals and plants. The remarkable point is 

 that while in most cases this embryonic growth is a direct and 

 simple process — e.g. animal and plant buds, embryonic de- 

 velopment of plant seeds — in some cases — e.g. most cases of 

 sexual reproduction of animals — it is a circuitous one, and the 

 embryonic phase shows stages of structure which seem to 

 possess a meaning other than that of being merely phases of 

 growth. 



As is well known, the explanation which is given of this 

 circuitous course of embryonic development is that we are 

 dealing with a special case of the law of heredity — "each 



