ALLAXTOIS AND URINARY BLADDER. 995 



The Allantois, Urinary Bladder, and UrachttB. The name of Allantois was 

 originally given to a membranous sac which is appended to the umbilicus of various 

 quadrupeds in the foetal state, and which communicates with the urinary bladder by 

 means of a canal passing through the umbilical aperture, and named the urachus. 

 These several parts are formed out of one original saccular process, which passes 

 out from the cloacal termination of the intestine, and which subsequently becomes 

 distinguished into the bladder, the urachus, and the allantois strictly so called ; 

 but modern embryologists employ the term allantois also to signify the original 

 common representative of the several parts referred to. In this sense, an allan- 

 tois may be said to exist not only in mammalia, but also in birds and reptiles, 

 subject, however, to great differences in its subsequent development and relative im- 

 portance. Thus, in Batrachians it never extends beyond the abdominal cavity ; in 

 scaly reptiles, on the other hand, as well as in birds and in some quadrupeds, it ulti- 

 mately surrounds the body of the foetus, and spreads itself over the inner surface of 

 the chorion or outer covering of the ovum, whilst in other quadrupeds, its extra- 

 abdominal portion is of small extent. In man, the allantois proper is not only very 

 insignificant in point of size, but also extremely limited in duration, for it vanishes 

 at a very early period in the life of the embryo ; and, whilst in many animals it serves 

 both as a receptacle for the secretion of the foetal urinary organs, and as a vehicle to 

 conduct the umbilical vessels from the body of the embryo to the chorion to form the 

 placenta (or some equivalent vascular structure), it seems in the human species to 

 serve merely for the latter purpose. The allantoid process communicates below with 

 the intestinal canal, and receives the wide excretory ducts of the Wolffian bodies, the 

 ureters, and the Fallopian tubes or vasa deferentia. By Baer, Rathke, and others, 

 the allantois has been regarded as formed from the intestinal tube, and by Reichert as 

 developed upon the excretory ducts of the Wolffian bodies. Bischoff states that, in 

 the embryos of the rabbit and dog, it commences before the appearance of either the 

 Wolffian bodies or the intestine, as a solid mass projecting forwards from the posterior 

 extremity of the body. This mass soon becomes hollowed into a vesicle, which is 

 covered with blood-vessels, and communicates with the intestine. Continuing 

 rapidly to enlarge, it protrudes between the visceral plates, and, when these close 

 together, through the opening of the umbilicus, forming in the rabbit a pear-shaped 

 sac, which conveys blood-vessels (soon recognised as the umbilical vessels) to the 

 chorion, to form the foetal part of the placenta. 



In the human embryo, the allantois ceases, at a very early period, to be found 

 beyond the umbilicus, and in the lower part of its extent, within the abdomen, it 

 becomes widened to form the bladder, whilst the upper part, or urachus, becomes 

 constricted, and is at length completely closed, and remains only in the form of 

 a ligament, with minute traces of its original hollow structure already described 

 along with the urinary bladder. 



The lower part of the allantois, or rudimentary bladder, receiving, as already men- 

 tioned, the efferent canals of the Wolffian bodies, as well as those of the kidneys ami 

 of the ovaries or testes, at first communicates freely with the lower end of the 

 intestinal tube, and when this becomes opened to the exterior, there is formed a sort 

 of cloaca, as in adult birds and reptiles. Soon, however, a separation takes place, 

 so as to produce for the genito-urinary organs a distinct passage to the exterior : thia 

 is named the sinus urogenitalis, and is situated in front of the termination of the 

 intestine. 



DEVELOPMENT OF THE ORGANS OF GENERATION. 



The development of the parts concerned in the reproductive function does not 

 begin until after the rudiments of the principal organs of the body have appeared. 

 The internal organs of generation first make their appearance, and for a brief period 

 no sexual difference is perceptible in them. The external organs, which subsequently 

 begin to be formed, are also identical in appearance in the two sexes, as late as the 

 fourteenth week. 



The internal Organs of Generation. The Ovaries and. Testes. The rudiments of 

 the ovaries or testes, for it cannot at first be determined which are ultimately to be 

 produced, appear after the formation of the allantois and Wolffian bodies, but a little 

 sooner than the kidneys. They consist of two small whitish oval masses of blastema 



3x2 



