Phylo genetic Considerations 647 



includes the chordates is much less clearly defined in its relationships. 

 Many of the chordate characters simply cannot be traced from begin- 

 nings in any of the invertebrate groups. This includes the all-important 

 characters such as the dorsal tubular nervous system, the notochord, and 

 the pharyngeal gill slits. 



From both biochemical and embryological evidence, it is now be- 

 lieved that the echinoderms and chordates had a common ancestral form. 

 This was probably a simply built, small, bilaterally symmetrical sessile 

 form which had a free-swimming dipleurula-like larva. From this hypo- 

 thetical ancestor, two lines developed. One led to the essentially ses- 

 sile, secondarily radially symmetrical echinoderms, the other to the per- 

 sistently bilaterally symmetrical, active chordates. 



HaeekePs Biogenetic Law.— During the tracing of the develop- 

 ment of the various phyla, it at times is clear that the adults of certain 

 phyla bear often striking resemblance to certain developmental stages in 

 the embryos of higher animals. This fact was evident even to early 

 biologists, but it was Haeckel (1834-1919), a German biologist, who 

 formulated the now famous biogenetic law or lazv of recapitulation. 



The euphonious phrase, "Ontogeny recapitulates phylogeny" briefly 

 states the essence of this theory, that is, the development of the indi- 

 vidual in a brief period goes through the development of the race. For 

 example, the human embryo passes from a one-celled stage, through the 

 morula, the blastula, and the gastrula. Each of these can be vaguely 

 compared to some living animal. At a later time, it has gill slits and a 

 notochord. It cannot be denied that such evidence indicates that there is 

 an element of truth in this theory. It must be pointed out, however, 

 that this is at best a half truth. Embryological stages may be shortened, 

 new structures may arise, and whole sections eliminated better to pro- 

 vide rapid formation of presumptive adult structures. This was empha- 

 sized during the discussion of such things as gastrulation in the mam- 

 malian egg. 



At times embryological stages are of value in showing the true re- 

 lationships of highly modified adults. Certainly the adult starfish would 

 not be considered a bilaterally symmetrical animal ; yet a study of its de- 

 velopment reveals that it is fundamentally a bilaterally symmetrical ani- 

 mal which has become highly modified. 



