So8 MANUAL OF ELEMENTARY ZOOLOGY 



of this narrow stalk in the middle of the belly. While the 

 amnion is being formed, a sac known as the allantois grows 

 out from the hinder part of the gut of the embryo. This 

 is lined with hypoblast and covered with splanchnic 

 mesoblast, and projects into the body cavity. It grows 

 down the stalk of the yolk sac and spreads out between the 

 true and false amnions. It becomes very vascular, and by 

 its means the embryo breathes through the porous shell. 

 The embryo chick has gill-clefts, and a system of arterial 

 arches like that of the tadpole, but never shows any trace 

 of gills. Finally, the beak pierces its way into an air 

 chamber which exists at one end of the egg between the 

 two membranes, and the animal begins to breathe by means 

 of its lungs. The allantois now shrivels up (the yolk sac 

 has already been absorbed) and the chick breaks its way 

 out of the egg. 



In all mammals except the Monotremata the egg is 

 minute and undergoes total segmentation. 

 ofiMammais. There is, however, no invagination such as is 

 found in Amphioxus, but a stage comparable to 

 the gastrula arises by the separation of layers of cells. 

 It is covered with a layer formed from the outer epiblast 

 cells known as the trophoblast. Outgrowths or " villi " 

 of this burrow into the wall of the uterus, in which the 

 embryo lies. The general course of development resembles 

 that of the chick, a yolk sac, amnion, and allantois being 

 formed. For a while the yolk sac forms a union with the 

 uterine wall and serves for nutrition and respiration, but 

 this is soon replaced by the allantois, which, as in the 

 chick, spreads out under the false amnion or subzonal mem- 

 brane and fuses with it. The organ thus formed is the 

 placenta, and from it outgrowths penetrate into the uterine 

 wall, expanding the original villi of the trophoblast and 

 obtaining nourishment and exchanging gases with the 

 maternal blood in lacunae which are formed around them 

 by the breakdown of blood vessels in the wall of the uterus. 

 The navel of the adult marks the site of the umbilical cord, 

 in which the stalk of the yolk sac and allantois and their 

 blood vessels entered the body. The amnion is the 

 " caul," and the placenta is shed as the " afterbirth." 



In the earthworm and the swan mussel the segmentation 



