DEVELOPMENT. 16r> 



that arc subordinated to lines rather than to points. And 

 in the higher organisms, considered as wholes, an arrange- 

 ment of parts in relation to an axis is distinct and nearly 

 universal. We see it in the superior orders of Thallophytes ; 

 and in all the cormophytic plants. With few exceptions the 

 Cfflenterata clearly exhibit it; it is traceable, though less 

 conspicuously, throughout the Mollusca; and the Annelida, 

 Arthropoda, and Vertebrata uniformly show it with perfect 

 definiteness. 



This kind of development, like the first kind, is of two 

 orders. The whole germ-product may arrange itself round 

 a single axis, or it may arrange itself round many axes : the 

 structure may be uniaxial or multiaxial. Each division of 

 the organic kingdom furnishes examples of both these 

 orders. In such Fungi as exhibit axial development 



at all, we commonly see development round a single axis. 

 Some of the Algce, as the common tangle, show us this 

 arrangement. And of the higher plants, many Monocotyle- 

 dons and small Dicotyledons are uniaxial. Of animals, the 

 advanced are without exception in this category. There is 

 no known vertebrate in which the whole of the germ-product 

 is not subordinated to a single axis. In the Arthropoda, 

 the like is universal; as it is also in the superior orders of 

 Mollusca. Multiaxial development occurs in most of 



the plants we are familiar with every branch of a shrub or 

 tree being an independent axis. But while in the vegetal 

 kingdom multiaxial development prevails among the highest 

 types, in the animal kingdom it prevails only among the 

 lowest types. It is extremely general, if not universal, 

 among the Ccelenteraia; it is characteristic of the Polyzoa; 

 the compound Ascidians exhibit it; and it is seen, though 

 under another form, in certain of the inferior Annelids. 



Development that is axial, like development that is central, 

 may be either continuous or discontinuous : the parts having 

 different axes may continue united, or they may separate. 



Instances of each alternative are supplied by both plants 

 12 



