'>5i 



CHORD ATA. 



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 tliis sac is an albuminous 



'^^'>^nrr^' 



Fig. 582. Fig. .58,3. 



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

 of pectoral flns. 



Fig. 583. — Embryonic envelopes of a mammal. (Diagram after Kolliker.) ah, amni- 

 otic cavity ; al, allantois ; am, amnion ; f?(y, yolk stalk ; ri.s, yolk sac ; c, embryo : 

 ft/i, ventral wall of embryo; r, extra-emVjryonic ccelom; 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 res23ira- 

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

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

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

 tlie serosa. 



Aristotle and liis followers recognized four divisions of vertebrates, and 

 these wore retained by Linne and Cuvier under tlie names Pisces, Reptilia 

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

 of tlieso into two classes, retaining the iinnie Reptilia for the one, Amphibia 

 for the other. Milne Edwards showed that this division corresponded 



