THE ALLANTOIS. 87 



and veins. When the allantois has assumed the form of a vesicle, it then 

 communicates both with the intestinal tube and the corpora Wolffiana, 



Fig. 13.* Fig. 14.t 



though the mode in which this communication is effected is not quite 

 clear. The allantois now rapidly increases in size, and the two umbilical 

 arteries in connection with it are recognised as branches of the iliac, 

 while the umbilical veins unite into either one or two trunks, which empty 

 themselves in the liver and the inferior vena cava. As the visceral 

 laminse close in the abdominal cavity, the allantois is thereby divided at 

 the umbilicus into two portions, the smaller of which is retained in the 

 abdomen, and is converted into the urinary bladder, while the external and 

 larger portion, accompanied by the umbilical vessels, proceeds to the 

 chorion, where its vessels are brought into connection with those developed 

 within the villi of this structure. The middle portion of the allantois, 

 that, namely, which traverses the umbilicus, at first contracts into a 

 canal, and subsequently is converted into a fibrous cord, the urachus.J 

 The different modes in which the allantois is subsequently disposed of 

 in the different orders of Mammalia, are described by Bischoff almost 

 as they were by Von Baer, whose account Bischoff for the most part 

 confirms. 



The development of the other parts of the Mammalian embryo will be 

 considered under the " Development of Organs," for, to avoid repetition, it 

 is deemed advisable to combine in one general account all the new infor- 

 mation on this subject, which has been obtained from observations on 

 different classes of animals. 



* Fig. 13. The lower part of the body of a bitch's embryo, magnified 10 diameters. 

 The mucous and vascular layers of the germinal membrane are turned back to shew (a) 

 the pedicle of the umbilical vesicle at its entrance into the abdominal cavity. b. I. Two 

 cellular masses out of which the allantois is formed. After Bischoff. v 



t Fig. 14. The lower extremity of an older embryo. The allantois a is developed into a 

 single vesicle, but its origin from two symmetrical halves is still shewn, especially by the 

 fissure in the middle. (Ibid.) 



J See Langenbeck's Account of the Allantois in the Human Ovum, on the next page. 



Muller's Physiology, p. 1570. 



