360 ESSAY ON CLASSIFICATION. 



V. Doubly Symmetrical type. (VERTEBBATA.) Evolutio Mgemina. The 

 development produces identical parts arising on both sides of an 

 axis, growing upwards and downwards, and shutting up along 

 two lines, so that the inner layer of the germ is inclosed below, 

 and the upper layer above. The embryos of these animals have 

 a dorsal cord, dorsal plates, and ventral plates, a nervous tube 

 and branchial fissures. 

 1. They acquire branchial fringes ; 



a. But no genuine lungs are developed. 



a. The skeleton is not ossified. CARTILAGINOUS FISIIES. 



$. The skeleton is ossified. FISHES PROPER. 

 I. Lungs are formed. AMPHIBIA. 



. The branchial fringes remain. SIRENS. 



/3. The branchial fringes disappear. UIIODELA and ANUIIA. 

 2. They acquire an allantois, but 

 . Have no umbilical cord ; 



a. Nor wings and air sacs. REPTILES. 



/8. But wings and air sacs. BIRDS. 



b. Have an umbilical cord. MAMMALIA. 



a. Which disappears early ; 



1. Without connection with the mother. MONOTREMATA. 

 2. After a short connection with the mother. MARSU- 



PIALIA. 



0. AVhich is longer persistent ; 



1. The yolk sac continues to grow for a long time. 

 The allantois grows little. RODENTIA. 

 The allantois grows moderately. INSECTIVORA. 

 The allantois grows much. CARNIVORA. 

 2. The yolk sac increases slightly. 



The allantois grows little. Umbilical cord very long. 



MONKEYS and MAN. 



The allantois continues to grow for a long time. Pla- 

 centa in simple masses. RUMINANTS. 

 The allantois continues to grow for a long time. Pla- 

 centa spreading. PACHYDERMS and CETACEA. 



CLASSIFICATION OF VAN BENEDEN. 



Van Beneden has also proposed a classification based upon Embryology, 

 which was first sketched in his paper upon the Embryology of the Bryozoa: 

 Recherches sur 1'anatomie, la physiologic, et 1'embryogenie des Bryozoaires; 

 Bruxelles, 1845, 4to. ; and afterwards extended in his Comparative Ana- 

 tomy : Anatomic comparee (Bruxelles, without date, but probably of the 

 your 1855), 1 vol. 12mo. 



