TO THE LOWER ANIMALS. 79 



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 ap- 

 plying itself to the walls of the cavity, in which the devel- 

 oping organism is contained, enables these vessels to be- 

 come 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 nour- 

 ishment and to get rid of effete matters, is termed the 

 ' Placenta? 



It would be tedious, and it is unnecessary for my pres- 

 ent 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 

 barn-door 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 



