BIOLOGICAL BEGINNINGS 33 



tion it has a disproportionately large head, as dissimilar to that of a dog 

 as the bud-like limbs are unlike his legs. 



The remains of the yelk, which have not yet been applied to the nutri- 

 tion and growth of the young animal, are contained in a sac attached to 

 the rudimentary intestine, and termed the yelk sac, or umbilical vesicle. 

 Two membranous bags, intended to subserve respectively the 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; sufKce it to say, that, by a long 

 and gradual series of changes, the rudiment here depicted and described, 

 becomes 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 pre- 

 cisely 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 animial; and this is built up round a 

 primitive groove, in the floor of which a notochord is developed. Further- 

 more, 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 differences between them are inconsiderable, while^ 



