112 ANIMAL BIOLOGY. [Parti. 



and of waste products from the foetus. These maternal and 

 foetal structures taken together constitute the organ known as 

 the placenta. 



While the placenta is being developed the folding off of the 

 embryo becomes more complete, there being (as seen in Fig 

 38, vi.) only a narrow vitelline duct connecting the remnant of 

 the umbilical vesicle with the embryo. Near the vitelline duct 

 is seen the duct of the allantois, and around both is the somatic 

 stalk of the amnion. The common stalk, consisting of vitelline 

 duct, allantoic duct, and enveloping amnion, is now known as 

 the umbilical cord. The mesoblast of the investing amnion stalk 

 develops a gelatinous tissue which cements together the whole 

 of the contents. The allantoic arteries in the cord in some 

 mammals wind in a spiral manner round the allantoic vein. 



At birth the foetal membranes, together with the uterine 

 mucous membrane with which they are so closely interlaced, are 

 delivered as the after-birth, such a placenta being called deciduate, 

 as opposed to non-deciduate where the foetal membranes separate 

 from the maternal. The mother gnaws through the umbilical cord 

 close to the embryo, and probably eats the placenta. 



The vitelline duct completely atrophies ; but the allantoic 

 stalk gives rise to two structures, (1) the urinary bladder, formed 

 by a dilatation of its proximal extremity ; (2) a cord known as 

 the urachus, connecting the bladder with the wall of the body at 

 the umbilicus. 



We have thus traced the rabbit to its birth. Let us now 

 return to the fowl. We have seen that the allantois grows out 

 at the posterior end of the embryo, and, applying itself closely 

 to the serous membrane, has a respiratory function. Thus does 

 the growing chick obtain the necessary oxygen. The necessary 

 food is obtained from the yolk-sac, the contents of which, how- 

 ever, do not pass directly into the -alimentary canal, for the 

 duct of the splanchnic stalk becomes early obliterated, but are 

 absorbed indirectly through the blood which circulates freely 

 in the vitelline arteries and veins. Owing to this absorption of 

 the yolk the yolk-sac gradually shrinks, and eventually, while 

 still of considerable size, is withdrawn into the body-cavity, 

 the walls of which gradually grow together at the umbilicus. 



At the broad end of the egg there is, between the two shell 



