AN INTRODUCTION TO ZOOLOGY 



similar to those in the chick. Despite the differences in its fate in 

 various forms it is always of importance, because its blood-vessels 

 form the vascular supply of the placenta and are very similar 

 throughout the mammalian series. The allantois appears as an 

 outgrowth of the hind gut and expands freely in the exoccel, which it 

 traverses and reaches the chorion in the trophodermic region. 

 It unites with this layer to form the allanto-chorion, and so almost 

 from its beginning forms a link between the embryo and the portion 

 of the blastoderm most intimately related to the uterine mucosa. 

 The vessels consist of a pair of umbilical arteries and a pair of 

 umbilical veins, and through them the chorionic region, previously 

 without vessels, becomes highly vascularised. This region is 

 destined to form the placenta, which therefore receives its embryonic 

 blood supply via the allantonic vessels, and it is important to re- 

 member that this is true, however small or large the actual allantois 

 may be. 



Placenta. 



The term placenta is applied to the organ that in the Mammalia 

 forms the connection between the lining wall of the maternal uterus 

 and the membranes surrounding the growing embryo. From the 

 entirely different mode of development in the chick it is obvious 

 that this structure is not represented in it. Structurally the 

 placenta is a complex, consisting of a very close apposition between 

 the foetal membranes and the uterine tissues, or more frequently 

 and typically it is an actual and complicated interpenetration of the 

 two that results in bringing their blood streams in close proximity 

 to one another. Functionally it serves in the early stages to anchor 

 the embryo to the uterine wall, and later it is the centre wherein the 

 dissolved salts and nutrient material of the parental blood can 

 transfuse into the foetal blood, thus providing the materials necessary 

 for growth. Also it provides a means whereby the excretory matters, 

 both nitrogenous substances and carbon dioxide, can diffuse from 

 the fcetal to the maternal blood, and so it also serves as an organ 

 of excretion. 



In the early stages of development the vascular walls of the yolk 

 sac may play a part in these functions, and so we speak of a yolk sac 

 or omphalopleural placenta, but sooner or later in all the higher 

 mammals, i.e. the Eutheria, an allantoic placenta is developed, and 

 this is one of the distinguishing characteristics of the order. 



We have already considered briefly different forms of behaviour 

 in the early implantation of the developing ovum ; it may be 

 central, or interstitial, or in certain cases it is eccentric, that is to say, 

 lies in a fold of the mucosa to one side. It is obvious that this will 



