EMBRYOLOGICAL SYSTEMS. 363 



upon different plans. There are probably not so many 

 different modes of development among plants as among 

 animals; unless the reproduction by spores, by naked 

 polyembryonic seeds, by angiospermous rnonocotyledonous 

 seeds, and by angiospermous dicotyledonous seeds, con- 

 nected with the structural differences exhibited by the 

 Acotyledones, Gymnospermese, Monocotyledones, and Di- 

 cotyledones, be considered as amounting to an indication 

 of different plans of structure. But, even then, these 

 differences would not be so marked as those which dis- 

 tinguish the four branches of the animal kingdom. The 

 limitation of classes and orders, which presents compa- 

 ratively little difficulty in the animal kingdom, is less 

 advanced among plants; whilst botanists have thus far 

 been much more accurate than zoologists in character- 

 izing families. This is, no doubt, chiefly owing to the 

 peculiarities of the two organic kingdoms. 



It must be further remarked that, in the classification 

 of Van Beneden, the animals united under the name of 

 Allocotyledones are built upon such entirely different 

 plans of structure, that their combination must of itself 

 satisfy any unprejudiced observer that any principle which 

 unites them in that way cannot be true to nature. 



DIAGRAM OF THE DEVELOPMENT OF ANIMALS BY 



KOLLIKER. 



KOLLIKER (A.), in his Entwickelungsgeschichte der Cephalopoden (Zurich, 

 1844, 1 vol., 4to , p. 175), has submitted the following diagram of the deve- 

 lopment of the animal kingdom. 



A. The embryo arises from a primitive part. (Evolutio ex una parte.} 



1. It grows in two directions, with bilateral symmetry. (Evolutio li- 



gemina.) 



a. The dorsal plates close up. VERTEBKATA. 

 I. The dorsal plates remain open and are transformed into limbs. 



AUTICULATA. 



