252 ESSAY ON CLASSIFICATION 



categories of the structure of animals had not been clearly distin- 

 guished.s® 



CLASSIFICATION OF K. E. VON BAER 



In conformity with his embryological investigations, K. E. von Baer proposes the 

 following classification. 

 I. Peripheric Type. (Radiata.) Evolutio radiata. The development proceeds from a 



centre, producing identical parts in a radiating order. 

 II. Massive Type. (Mollusca.) Evolutio contorta. The development produces identical 

 parts curved around a conical or other space. 



III. Longitudinal Type. (Articulata.) Evolutio gemina. The development produces 



identical parts arising on both sides of an axis and closing up along a line op- 

 posite the axis. 



IV. Doubly Symmetrical Type. (Vertebrata.) Evolutio bigemina. The development 



prociuces 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 Fishes. 

 /3. The skeleton is ossified. Fishes proper. 



b. Lungs are formed. Amphibia. 



a. The branchial fringes remain. Sirens. 

 ^. The branchial fringes disappear. Urodela and Antira. 

 2°. They acquire an allantois, but 



a. Have no umbilical cord; 



a- Nor wings and air sacs. Reptiles. 

 p. 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. Marsupialia. 

 p. Which 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. Placenta in simple 



masses. Ruminants. 

 The allantois continues to grow for a long time. Placenta spreading. 

 Pachyderms and Cetacea. 



CLASSIFICATION OF VAN BENEDEN 

 Pierre J. Van Beneden has also proposed a classification based upon Embryology, 

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

 I'analomie, la physiologic ct I'euibryogenie des Bryozoaires . . . (Brussels, 1845) (pam- 

 phlet), and afterwards extended in his Anatomie comparee (Brussels, without date, 

 but probably from the year 1855). 



™ Compare Chap. II, Sect, i to ix. 



