134 THE INDUCTIONS OF BIOLOGY. 



minute structure. Let us glance at these propositions in the 

 concrete. Leaving out those Rhizopods which are 



wholly structureless, every plant and animal in its earliest 

 stage, consists of a spherical sac, full of liquid containing 

 organic matter, in which is suspended a nucleated cell, more 

 or less distinct from the rest ; and the first changes that 

 occur in the germ thus constituted, are changes that take 

 place round centres produced by division of the original 

 centre. From this type of structure, the simplest organisms 

 do not depart ; or depart in no definite or conspicuous ways. 

 Among plants, the TTredo and the several tribes of Protococci 

 permanently maintain such a central distribution ; while 

 among animals, it is permanently maintained by crea- 

 tures like the Gregarina, and in a different manner by the 

 Amceba, Actinophri/s, and their allies. In larger organisms, 

 made up chiefly of units that are analogous in structure to 

 these simplest organisms, the formation of units ever continues 

 to take place round points or nuclei ; though the arrangement 

 of these units into groups and wholes may proceed after 

 another method. 



Central development may be distinguished into unicentral 

 and multicentre^ ; according as the product of the original 

 germ, develops symmetrically round one centre, or develops 

 without subordination to one centre — develops, that is, in 

 subordination to many centres. Unicentral de- 



velopment, as displayed not in the formation of single cells 

 but in the formation of aggregates, is not common. The 

 animal kingdom shows it only in the small group named 

 llialassicollce : inert, spherical masses of jelly, with scarcely 

 any organization, which are found floating in southern seas. 

 It is feebly represented in the vegetal kingdom by the Vol- 

 vox globator. On the other hand, multicentral devel- 



opment, or development round insubordinate centres, is va- 

 riously exemplified in both divisions of the organic world. It 

 is exemplified in two distinct ways, according as the insubor- 

 dination among the centres of development is partial or total 



