THE ALL AN TO IS. Ill 



course of the Wolffian duct is changed during fetal life by the migration of the 

 testis from its original abdominal position into the scrotum. In the female the 

 Wolffian duct becomes vestigial. 



The Mullerian ducts develop much later. They may be found in the 12 mm. 

 pig as two short funnels formed by the mesothelium and situated on the ventral 

 side of the mesonephros near the septum transversum (diaphragm). The funnels 

 point backward and grow into tubes, which run on the ventral side of the Wolffian 

 duct and presently connect with and open into the base of the allantois. The 

 pelvic portions of the two Mullerian ducts approach one another in the median 

 line and in the female they fuse, making a median epithelial canal, the anlage of 

 the uterus and vagina. The original entrance to the canal persists as the fimbriate 

 opening and the stretch of the original canal between the funnel and the uterus 

 becomes the uterine tube. 



The Allantois. 



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

 by entodermal epithelium (Fig. -21). It arises on the ventral side of the caudal end 

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

 guish two main types. The first type is illustrated by the sauropsida and the un- 

 gulates. 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 chick embryos, 

 for by the fourth day it has become a considerable rounded vesicle which lies 

 in the extra-embryonic ccelom between the yolk-sac and the extra-embryonic soma- 

 topleure 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 enor- 

 mous. 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 unguiculate 

 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 (Fig. 69, b.s). Into this body-stalk the diverticulum consti- 

 tuting the allantois extends (Fig. 80, All, and Fig. 25, All). The entoderm of 

 the allantois is surrounded by mesoderm, which is present in the body-stalk in con- 

 siderable 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. 64). 

 By means of the body-stalk a connection is established between the embryo and 



