136 THE INDUCTIONS OF BIOLOGY. 



mensiojQs — have an obscure if not a distinct axial structure. 

 The originally cellular units out of which higher organisms 

 are mainly built up, usually pass into shapes that are subordi- 

 nated to lines rather than to points. And in the higher organ- 

 isms, considered as wholes, an arrangement of parts in rela- 

 tion to an axis is distinct and nearly universal. We see it in 

 the superior orders of Thallogens ; and in all the Acrogens, 

 Endogens, and Exogens. With few exceptions the Ccelente- 

 rata clearly exhibit it ; it is traceable, though less conspicu- 

 ously, throughout the Mollusca; and the Annulosa and 

 Vertehrata 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 miiltiaxial. Each division of 

 the organic kingdom furnishes examples of both these or- 

 ders. In such Fangi 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 arrange^ 

 ment. And of the higher plants, many Endogens and 

 small Exogens are uniaxial. Of animals, the advanced are 

 without exception in this category. There is no known ver- 

 tebrate in which the whole of the germ-product is not subor- 

 dinated to a single axis. In the more fully-organized Anmi- 

 losa, the like is almost 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 tho 

 highest types; in the animal kingdom, it prevails only among 

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

 among the Ccelenterata ; it is characteristic of the Mollus- 

 coida ; among Molluscs the compound Ascidians exhibit it ; 

 and it is seen, though under another form, in the inferior 

 Annulosa, 



Development that is axial, like development that is centralt 



