I INTRODUCTORY 3 



organs the endoderm of a Vertebrate, for instance, contains the 

 material for the alimentary tract and its derivatives gill-slits, 

 lungs, liver, bladder, and the like ; the germ-layers are therefore 

 not ultimate but elementary organs, and elementary organs of 

 the first order. In the next stage these primary organs become 

 subdivided into secondary organs as the archenteron of an 

 Echinoderm becomes portioned into gut and coelom-sac, or the 

 ectoderm of an Earthworm into epidermis, nervous system, and 

 nephridia and in subsequent stages these again become suc- 

 cessively broken up into organs of the third and fourth orders 

 and so on, until finally the ultimate organs or tissues are formed, 

 each with special histological characters of its own. This end 

 is, however, not necessarily reached by all the tissues at the 

 same time. Indeed, it is no uncommon thing for certain of 

 them to attain their final structure while the others are yet in 

 a rudimentary condition ; thus, in some Sponges the scleroblasts 

 begin to secrete spicules in the larval period, nematocysts may 

 be formed in the Planula of the Coelenterates, notochordal tissue 

 is differentiated in the newly hatched tadpole of the Frog ; and, 

 speaking generally, larval characters are developed at a very 

 early stage. 



To this regular sequence of ontogenetic events Driesch has 

 applied the term ' rhythm ', the rhythm of development. The 

 organs of the body are, however, by no means all formed of single 

 tissues bone, epithelium, blood, and the rest but are com- 

 pounded, frequently of very many tissues, and this ' composition ', 

 to quote a term of Driesch's again, is another of the obvious 

 features of organogeny. 



While, therefore, in the last resort all differentiation is histo- 

 logical, that final result, the assumption by the cells of their 

 definitive form, is only achieved after many changes have taken 

 place in the position of the parts relatively to one another while 

 the organs are being compounded, and so its specific shape 

 conferred upon the whole body. 



It is possible to find a few general expressions for the manifold 

 changes that take place in the relative positions of the parts. 

 Several years ago, in 1874, His compared the various layers of 

 the chick embryo to elastic plates and tubes; out of these he 



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