II FCETAL APPENDAGES 87 



The remains of the yelk, which have not yet 

 been applied to the nutrition 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 pro- 

 tection 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 con- 

 tained, 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 inter- 

 lacement 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, becomes a puppy, is 



