554 



CHORD AT A. 



lantoidea, from the absence of these structures. The amnion is a 

 sac which envelops the whole embryo and is connected with the 

 rest only at the umbilicus, that is, the point where the yolk sac 

 projects from the ventral wall. In this sac is an albuminous 



FIG. 582. Fm. 583. 



FIG. 582. Shark embryo. (From Boas.) y, part of yolk sac ; y, external gills in front 



of pectoral fins. 

 FIG. 583. Embryonic envelopes of a mammal. (Diagram after Kolliker.) aft, amni- 



otic cavity ; al, allantois ; aw, amnion ; da, yolk stalk ; ds, yolk sac ; e, embryo : 



hh, ventral wall of embryo; r, extra-embryonic coelom; sft, serosa ; sz, serosal 



villi. 



amniotic fluid. The amnion is genetically a part of the ventral 

 surface; it develops ventrally as folds lateral, anterior, and pos- 

 terior which grow up over the back on all sides and unite above 

 the embryo. 



The allantois is an enlargement of the urinary bladder. This 

 grows out from the body cavity at the umbilicus and extends be- 

 tween yolk sac and amnion and then grows in all directions until 

 its folds meet above the back. The part of the allantois which re- 

 ceives the urine may be enlarged or not. The rest of the out- 

 growth consists of blood-vessels and connective tissue. The blood- 

 vessels are the most important, for the allantois forms the respira- 

 tory apparatus of the embryo, and in the mammals it develops the 

 placenta, by which nourishment as well is conveyed to the young. 

 Yolk sac, amnion, and allantois are enveloped in a common coat, 

 the serosa. 



Aristotle and his followers recognized four divisions of vertebrates, and 

 these were retained by Linne" and Cuvier under the names Pisces, Reptilia 

 or Amphibia, Aves, and Mammalia. Blainville (1818) divided the second 

 of these into two classes, retaining the name Reptilia for the one, Amphibia 

 for the other. Milne Edwards showed that this division corresponded 



