DEVELOPMENT. 163 



concrete. Practically every plant and every animal 



in its earliest stage is a portion of protoplasm, in the great 

 majority of cases approximately spherical but sometimes elong- 

 ated, containing a rounded body consisting of specially modi- 

 fied protoplasm, which is called a nucleus; and the first changes 

 that occur in the germ thus constituted, are changes that take 

 place in this nucleus, followed by changes round the centres 

 produced by division of this original centre. From this type of 

 structure, the simplest organisms do not depart ; or depart in 

 no definite or conspicuous ways. Among plants, many of the 

 simplest Algce and Fungi permanently maintain such a central 

 distribution; while among animals it is permanently main- 

 tained by creatures like the Gregarina, and in a different man- 

 ner by the Amoeba, Actinophrys, and their allies: the irregu- 

 larities which are many and great do not destroy this general 

 relation of parts. In larger organisms, made up chiefly of 

 units that are analogous to these simplest organisms, the for- 

 mation of units ever continues to take place round nuclei; 

 though usually the nuclei soon cease to be centrally placed. 



Central development may be distinguished into unicentral 

 and muUicentral ; according as the product of the original 

 germ develops more or less symmetrically round one centre, 

 or develops without subordination to one centre — develops, 

 that is, in subordination to many centres. Unicentral 



development, 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 some of the small group of 

 colonial Radiolaria. It is feebly represented in the vegetal 

 kingdom by a few members of the Volvocinece. On the 



other hand, multicentral development, or development round 

 insubordinate centres, is variously exemplified in both divi- 

 sions of the organic world. It is exemplified in two distinct 

 ways, according as the insubordination among the centres of 

 development is partial or total. We may most conveniently 

 consider it under the heads hence arising. 



Total insubordination among the centres of development. 



