CHANGES IN THE OVUM. 

 AH.AKTOIS. 



77 



The allantois arises on the front part of the posterior extremity of the 

 mucous layer which is closing to form the intestine, as a growth of the 

 intestine, which proceeds very rapidly. It passes out where the ventral 

 lamina are still unclosed, in the region of the umbilicus, and reaches, 

 either mediately or immediately, the inner surface of the exochorion. 

 By the constriction of the umbilicus, it is separated into two portions 

 which communicate : that within the body of the embryo is the sacculated 

 urinary bladder, with the urachus or tube of communication. It receives 

 its vessels from the hypogastric artery ; these are spread out as a vascular 

 layer, especially upon that portion of its surface which faces the exochorion. 

 The vessels form a distinct layer — the endochorion. As a membrane it 

 lines the inner face of the chorion, and is reflected around the point of 

 insertion of the umbilical cord, to be spread over the outer face of the 

 amnion ; in this way it transforms the chorionic sac into a kind of serous 

 cavity, in which the amniotic sac is enclosed like a viscus. 



Fig. 40. 



Plan of the Fcetal Envhlopes in the Mare. 



A, Foetus; B, Cavity of the Amnion; C, Cavity of the Allantois; D, Umbilical Vesicle: 

 1, Chorion ; 2, Chorial Layer of the Allantois ; 3, Amniotic Layer of the Allantois ; 4, 

 Amnion ; 5> Urachus, the expansion of which forms the Allantois ; 6, Pedicle of the Um- 

 bilical Vesicle. 



The inner, or amniotic layer, is so loosely united to the amnion, that a 

 slight dissection or inflation will readily detach it. When inflation is 

 practised, the separate membrane presents a wavy appearance, from the 

 presence of numerous cellular attachments it has with the amnion ; as the 

 inflation is forced these bands tear with a noise like the crackling of 

 parchment, and with care the whole of this portion, which is equal in 

 extent to the amnion, may be removed. The chorial allantois adheres much 

 more firmly, and in some parts it can scarcely be dissected away. Infla- 

 tion, however, demonstrates its existence and continuity with that on the 

 amnion ; for if, after opening the allantoid sac by cutting through the 

 chorion and the layer covering it, we introduce a tube between the two 

 membranes — which is easily done near a large vessel, a slight inflation 

 causes the air to penetrate between the allantois and chorion, though 

 only in the track of the vessels of a certain size, where the adherence is 

 slight ;• a more powerful inflation will cause the air to follow the smaller 



