FCETAL ENVELOPES 



381 



described above. Little is kntown of the phylogeny of the amniotic structures, 

 which are without parallel in the animal kingdom, except in the scorpions where 



Fig. 404. — Diagrams of the development of amnion and allantois. Upper figure 

 earlier, transverse section; lower later, longitudinal, a, amnion; al, alimentary canal; 

 ym., cav, amniotic cavity; ch, beginning of placental area; s, serosa; so, somatoplcure; 

 at, yolk stalk. 



wmilar structures arise in the same way. Of course there is no phylogenetic 

 connexion between the two. It has been suggested that there may be a tendency 



