464 Causes and Course of Organic Evolution 



marine Malacohdella and Ahrostomum still living. In the 

 latter cases, and in types related that may once have been 

 abundant, we have to look, it seems, for the more typical 

 notochordal extension seen in craniates. For in such a case 

 when the sheath filled up with central occluding tissue its 

 anterior extension would reach only to the mouth and the 

 hinder part of the brain. Round this as a mechanically jflexible 

 but yet resistant rod, added strengthening substance would 

 naturally center, as new and varied stimuli became added. 



So we have a continuous notochord in the cyclostomes, 

 that bears only cartilaginous neural arch processes, and occa- 

 sionally ventral processes as well. In the Apoda however 

 these arches are not cartilaginous but bony, and become con- 

 tinuous round the notochord to form bony amphicoelous rings 

 or vertebrae. The biconcave or pseudocoelous vertebrae thus 

 formed are the simplest type in construction, though they 

 are a marked advance on the conditions in cyclostomes. But 

 further in Apoda cartilaginous material is laid down round 

 the fibrous notochord sheath between each osseous ring, and 

 even grows inward within each biconcave ring, so as to absorb 

 there the whole notochordal substance. Thus the originally 

 continuous notochord with its fibrous sheath and cartilaginous 

 hoops, as seen in cyclostomes, becomes by degrees an alter- 

 nating set of cartilaginous rings and osseous biconcave rings 

 with hollow centers, that surround the notochord; while ulti- 

 mately as maturity is reached the cartilage,, having grown 

 into the hollow centers of the osseous vertebrae, where only 

 notochordal substance was, replaces that substance entirely. 



A transition is made from the Apoda through the simpler 

 urodeles like Siren and Amphiuma to more advanced types 

 like Desmognathus and from it to Diemyctylus and Salamandra, 

 in that the pseudocoelous (amphicoelous) osseous rings become 

 more and more lengthened, the cartilage between more and 

 more narrowed, and the substance of the notochord more 

 and more completely replaced by bone, till in groups that are 

 represented by the three last named genera the most complex 

 condition is reached. Here each osseous vertebra has its 



