250 ON THE RELATIONS OF MAN 



protection and nutrition of the young creature, have 

 been developed from the skin and from the under and 

 hinder surface of the body ; the former, the so-called 

 ' amnion,' is a sac filled with fluid, which invests the whole 

 body of the embryo, and plays the part of a sort of water- 

 bed for it ; the other, termed the ' allantois,' grows out, 

 loaded with blood-vessels, from the ventral region, and 

 eventually applying itself to the walls of the cavity, in 

 which the developing organism is contained, enables these 

 vessels to become the channel by which the stream of 

 nutriment, required to supply the wants of the offspring, 

 is furnished to it by the parent. 



The structure which is developed by the interlacement 

 of the vessels of the offspring with those of the parent, 

 and by means of which the former is enabled to receive 

 nourishment and to get rid of effete matters, is termed 

 the ' Placenta.' 



It would be tedious, and it is unnecessary for my present 

 purpose, to trace the process of development further ; 

 suffice it to say, that, by a long and gradual series of 

 changes, the rudiment here depicted and described be- 

 comes a puppy, is born, and then, by still slower and less 

 perceptible steps, passes into the adult Dog. 



There is not much apparent resemblance between a 

 barndoor Fowl and the Dog who protects the farm-yard. 

 Nevertheless the student of development finds, not only 

 that the chick commences its existence as an egg, primarily 

 identical, in all essential respects, with that of the Dog, 

 but that the yelk of this egg undergoes division that the 

 primitive groove arises, and that the contiguous parts of 

 the germ are fashioned, by precisely similar methods, into 

 a young chick, which, at one stage of its existence, is so 

 like the nascent Dog, that ordinary inspection would 

 hardly distinguish the two. 



The history 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 yelk of that 

 egg always undergoes division, or ' segmentation ' as it is 

 often called: the ultimate products of that segmentation 

 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. 



