CH. LIX.] THE ALLANTOIS 843 



in the embryo at the umbilical orifice. In the later periods it is entirely 

 obliterated, for the amnion is distended till its outer surface fuses 

 with the inner surface of the chorion ; and at the same time the 

 umbilical cord is differentiated as the distending amnion surrounds 

 and presses together the allantoic stalk and the remains of the yolk- 

 sac (fig. 634). 



At birth, on account of the contraction of the walls of the uterus 

 and the pressure of the surrounding muscles, the liquor amnii forces 

 part of the membrane formed by the fused amnion and chorion 

 through the cervix uteri, which is gradually distended. When the 

 distension is sufficient, the membrane ruptures, the liquor amnii 

 escapes, and afterwards the child is forced out. It still remains con- 

 nected with the placenta by the umbilical cord, and this connection 

 should not be severed for a few minutes, in order that as much blood 

 as possible may be aspirated from the foetal part of the placenta into 

 the child as breathing commences. 



After the child is expelled the contraction of the uterine wall con- 

 tinues and the placenta is separated and forced out. The separation 

 gradually extends through the decidua, along the line of the stratum 

 spongiosum, and the fused chorion amnion and decidua turned inside 

 out, follow the placenta to which they are attached, constituting, with 

 the placenta, the after-birth. 



After the umbilical cord is tied and separated, the umbilical 

 arteries inside the child become filled with blood-clot, and ultimately 

 they are converted into fibrous cords, the so-called obliterated hypo- 

 gastric arteries, and at the same time the allantois is also converted 

 into a fibrous strand, the urachus, which extends from the apex of the 

 bladder to the umbilicus. 



Development of the Framework of the Body. 



In the early stages of development, the only indication of a frame- 

 work or skeleton is the notochordal rod of hypoblastic cells, which 

 extends along the whole length of the dorsal wall of the primitive 

 intestine beneath the neural tube, its anterior end being situated 

 immediately behind the position where the pituitary body is after- 

 wards formed. In mammals the notochord disappears, except in the 

 centres of the intervertebral discs, but in amphioxus and lampreys it 

 persists as a permanent skeletal support, and in these cases it closely 

 resembles cellular cartilage enclosed in a fibrous sheath. It is com- 

 posed of a very insoluble proteid-like substance, which, however, is 

 not collagen. Collagen and gelatin (which is formed from collagen by 

 boiling), are characteristic of true connective tissues which are formed 

 from mesoblast; the notochord is hypoblastic. The notochord 

 contains also, like all embryonic tissues, a large quantity of glycogen. 



