THE ALLANTOIS. 103 



Further details are given in the practical part in connection with the study of 

 the embryo pig and chick. 



The third of the excretory organs is termed the metanephros or true kidney. 

 It exists in all adult amniota, but only in them. In development and in struc- 

 ture it differs very much from the other excretory organs. For an account of its 

 origin in mammals, see page 257. 



It is essential that the student of embryology should have a clear prelimi- 

 nary notion of these organs, for without such he will be unable to comprehend an 

 important series of embryological phenomena. 



The Allantois. 



The allantois is a diverticulum of the entodermal canal, and is, therefore, 

 lined by entodermal epithelium. It arises on the ventral side of the caudal end 

 of the embryo in proximity to the anal plate. In its development we can dis- 

 tinguish two main types. The first type is illustrated by the sauropsida and the 

 ungulates. In them it grows out and rapidly enlarges so as to form a vesicle of 

 considerable size and connected with the embryo by means of a narrow, hollow 

 stalk. When the allantois develops according to this type, it is spoken of as free, 

 because it has no connection with the extra-embryonic somatopleure (chorion 

 and amnion). This form of the allantois may be readily observed in chicken 

 embryos, for by the fourth day it has become a considerable rounded vesicle 

 which lies in the extra-embryonic coelom between the yolk-sac and the extra- 

 embryonic somatopleure or membrana serosa. During the fifth day it rapidly 

 enlarges, and at the beginning of the sixth day is nearly or quite as large as the 

 head of the embryo. In ungulates the growth of the free allantois begins very 

 early and becomes enormous. Its principal expansion is sideways, that is to say, 

 at right angles to the axis of the embryo, and it becomes a large sac, very much 

 larger, indeed, than the entire embryo. 



The second type of allantois occurs in the placental mammals of the unguic- 

 ulate series and is not known to occur in any species of the ungulate type. In 

 probably all unguiculates the posterior end of the body has a prolongation which 

 is known as the body-stalk. Into this body-stalk the diverticulum constituting 

 the allantois extends. The entoderm of the allantois is surrounded by mesoderm , 

 which is present in the body-stalk in considerable volume. On the outer surface 

 there extends a layer of ectoderm, so that the three germ-layers enter into the 

 formation of the body-stalk as they do into the formation of the embryo. These 

 relations are illustrated by the diagram (Fig. 48). By means of the body-stalk 

 a connection is established between the embryo and the extra-embryonic soma- 

 topleure or primitive chorion, Cho. Later, when the formation of the amnion is 



