620 COMPARATIVE ANATOMY. 



This relation is henceforward the common one ; in the Amphibia, 

 Reptilia, and Aves, there is a cloaca of pretty much the same kind ; 

 in Birds it is provided with a diverticulum, the bursa Fabricii 

 (Fig. 333, h), which is attached to its hinder wall. The cloaca must 

 be regarded as being inherited by all the Mammalia, although it is 

 in the Monotremata only that it persists without any great modifi- 

 cation ; in the rest it undergoes considerable changes. The most 

 important of these is the share which it takes in the differentiation 

 of a copulatory organ, and which was faintly indicated in the Am- 

 phibia ; these changes end by giving rise to a urogenital orifice 

 distinct from the anus. The allantois is one of the most im- 

 portant of the organs which are differentiated from the cloaca ; it is 

 developed from the anterior wall of the cloaca, that is, from the 

 part of the primitive cavity of the hind-gut that represents it. In 

 Lepidosiren and in the Amphibia this organ forms a body which 

 springs from the anterior wall of the cloaca by a short stalk ; in the 

 latter it is continued into two anteriorly placed diverticula ; it lies 

 freely in the ccelom. It is known as the urinary bladder, and 

 seems indeed to function as such, although the ureters open some 

 way from it. Blood-vessels are distributed on its thin wall ; the 

 arteries come from the pelvic vessels, and the veins pass to the portal 

 vein. 



In the Amniota this organ is very greatly developed during the 

 embryonic stages, and becomes a large sac which grows out far 

 beyond the embryo, and is provided with a large number of vessels ; 

 it envelops the embryo, already covered by the amnion. In 

 Keptiles and Birds it gradually atrophies as the abdominal wall is 

 closed in, and disappears altogether, In the Saurii and Chelonii 

 only the portion of the allantois within the abdominal cavity is 

 retained ; in them it is widened out into a sac, which is provided 

 with diverticula on either side (Fig. 349, z>). 



In the Mammalia this organ has different relations to the develop- 

 ing organism. As in the Reptilia and Aves it grows out into a 

 vesicle, which communicates with the cavity of the hind-gut by a 

 stalk which runs inside the umbilical chord. That portion of the 

 chord, which passes into the ccelom (urachus) is partly converted 

 into a ligament (Lig.vesico-umbilicale medium), partly into the urinary 

 bladder, and partly into a sinus urogenitalis, where the orifices of 

 the generative ducts pass into it. In the Monotremata and Marsupialia 

 the peripheral portion appears to have the same relations as in the 

 Sauropsida., while in other Mammals it aids in the formation of the 

 "chorion," which is connected by villous elevations with the mucous 

 membrane of the uterus. When these vascular villi of the chorion 

 are further developed, the foetal blood passing along the vessels of the 

 allantois, acquires a distribution in the peripheral regions of the sac. 

 This effects exchanges with the blood which is distributed in the 

 mucous membrane of the uterus. As it becomes more intimately 

 connected with the uterine mucous membrane a placenta is 

 developed ; this varies greatly in character according to the way 



