590 DEVELOPMENT [CHAP. XLI. 



tube which is a little twisted, and gradually becomes completely 

 bent upon itself. The development of the heart may be watched 

 in the chick from the beginning of the second day of incubation. 

 It appears between the mucous and serous laminae in the form of 

 a sac, terminating in two or three branches, the future venous 

 trunks. At this early period, even rythmical motions may be 

 observed, but the fluid in its anterior is at this time nearly colour- 

 less. The large vascular trunk immediately after its origin from 

 the ventricle divides into four vascular arches, which unite again 

 to form the aorta. This vessel, as it passes down close to the 

 spine, again divides, and gives off transverse branches; some of 

 which, the arteriae omphalo-mesentericse, lying upon the duct of 

 the same name, are conducted to the yolk-membrane, upon the 

 surface of which they are spread out. These vessels absorb the 

 nutrient constituents of the^yolk, and carry them to the system 

 of the foetus. With their corresponding veins, they form a net- 

 work over the umbilical vesicle. Their arrangement will be more 

 fully described in chapter xliii., on the development of the mem- 

 branes of the foetus. 



Allantois. The allantois is first observed as a little solid 

 eminence, composed of cells upon the anterior surface of the caudal 

 extremity of the embryo (Figs. 279, 280 g.). A cavity is soon 

 formed, which is continuous at one period with the lower part of 

 the intestine. 



The allantois grows very rapidly, and in ruminants soon sur- 

 rounds, the. entire foetus, its outer surface being in close contact 

 with the chorion; but in rodent animals, and in man, its chief 

 office is probably that of conducting the vessels of the foetus to the 

 chorion. 



r 



DEVELOPMENT OF THE HUMAN EMBRYO. 



It was formerly supposed that man, in his development, passed 

 through various successive stages, each of which was said to have 

 its permanent representative among the lower animals; but a 

 doctrine so obviously untrue has long since ceased to have any 

 supporters. Von Baer showed the fallacy of such a statement; 

 but at the same time proved that, up to a certain period of 

 development, the changes occurring in the human embryo were 

 precisely similar to those which take place in the development of 

 all other vertebrate animals ; that, for instance, up to a certain 

 period of its existence, we should be unable to distinguish the 

 embryo of man, or one of the higher vertebrata, from that of a 



