490 



CHORDATA. 



discoidal segmentation, and is the result of the accumulation of the mate- 

 rial in the digestive tract, which forces out its ventral wall like a hernia. 

 Its presence in the mammals, which have small eggs lacking in yolk, is an 

 indication that these have descended from large-^^olked forms, such as 

 the monotremes yet are. The embryo either lies directly on the yolk or is 

 connected with it by a yolk stalk. 



AMiile the yolk sac is widely distributed, the amnion and allantois are 

 restricted to reptiles, Ijirds, and mammals, which are consequently spoken 

 of as Amniota or AUantoidea, in contrast to the fishes and Amphibia, 

 which are frequently caled Anamnia or Anallantoidea, from the aljsence 

 of these structures. The amnion (fig. 541, am) is a sac which envelops 



Fig. 540. Fig. 541. 



Fig. 540. — Shark embrj'O (from Boas), y, part of yolk sac; g. external gills in front 

 of pectoral fins. 



Fig. 541. — Embrj'onic envelopes of chick (schematized, after Duval), a/, allan- 

 tois; am, amnion; ait eye; c, extraembryonic ccelom; d, digestive tract; do, yolk sac; k, 

 gill clefts; m, mid-brain; i, 2, hinder and anterior extremities. 



the whole emlsryo and is connected with the rest only at the umbilicns, 

 that is, the point where the yolk sac projects from the ventral wall. In 

 this sac is an albuminous amniotic Jhi id. The amnion is genetically a 

 part of the ventral surface ; it develops ventrally as a circular fold — lateral, 

 anterior, and posterior — which grows up over the back on all sides and 

 unites above the embryo. 



The allajitois (al) is an enlargement of the urinary bladder. This 

 grows out from the Ijody cavity at the umbilicus and extends between 

 yolk sac and amnion and then grows in all directions until its folds meet 

 aljove the back carrying with it blood-vessels and connective tissue. The 

 blood-vessels are the most important, for the allantois forms the respiratory 



