THE FCETAL APPENDAGES. 23 



axis) develop into the walls of chest and abdomen; farther 

 out it turns up and arches over the back of the embryo, and 

 its edges, there meeting, grow together and form a bag, the 

 am-nion, enveloping the foetus. Into this a considerable 

 quantity of liquid is secreted, in which the foetus floats. At 

 birth the contractions of the uterus, pressing on the amnion, 

 drive part of it down like a wedge into the neck of the 

 uterus, and through its liquid contents an equable pressure 

 is exerted there, until the os uteri is tolerably dilated; the 

 sac then normally ruptures and the "waters" escape. Some- 

 times, however, an infant is born still enveloped in the 

 amnion, which is then popularly known as a caul. While 

 the amnion is developing, a semi-cartilaginous rod forms 

 along the axis of the Body beneath the floor of the dorsal 

 tube; this is the notochord; when it appears the young being 

 is marked out distinctly as a vertebrate animal, having a dor- 

 sal neural tube above an axial skeleton, and a ventral licemal 

 tube, (p. 4), formed by the proximal regions of the somato- 

 pleure, beneath it. The ventral tube, however, is still widely 

 open, the points where the amniotic folds turn back being 

 far from meeting in the future middle line of the chest and 

 abdomen. 



The proximal portions of the splanchnopleure incurve to 

 inclose the alimentary tube, which is at first straight and 

 simple. Beyond the point where it bends in for this purpose 

 the splanchnopleure again diverges, and incloses a small 

 globular bag, the yelk sac, which is, thus, attached to the 

 ventral side of the alimentary canal; it at first projects 

 through the opening where the amniotic folds turn back, 

 but has little importance in the mammalian embryo and is 

 soon absorbed. 



The allantois is primarily an outgrowth from the ali- 

 mentary canal, containing blood-vessels. It passes out from 

 the Body on the ventral side where the somatopleures have 

 not yet met, and reaching the inside of the uterus, its distal 

 end expands there to make the main part of the placenta (see 

 below). Its narrow proximal portion forms the umbilical 

 cord, around which the somatopleures, incurving to in- 

 close the belly, meet at the navel some time before birth. 



