CHAPTER XVIII. 

 THE MIDDLE CORD THE LEMMATOCHORD AND THE NOTOCHORD. 



The failure to recognize the notochord in invertebrates has been due to the 

 prolonged domination of the annelid and gastrula theories. 



It has been generally assumed that the notochord is found only in the verte- 

 brates because no one could find one in the invertebrate midgut where the gas- 

 trula theory proclaims it ought to be located if present. So long as- it was con- 

 fidently assumed that the " archenteron " of vertebrates was the ontogenetic 

 repetition of an ancestral midgut, and that its associated parts, mesoderm and 

 notochord, were necessarily entodermic in origin, no one ventured to look for the 

 notochord elsewhere than in the midgut of an annelid, or in some other worm- 

 like invertebrate. It is not surprising then that almost any unpaired enteric 

 outgrowth has been called, at one time or another, a notochord, thus giving a 

 pseudo-respectability to such morphological curiosities as diplochorda, adelo- 

 chorda, hemichordata^ etc. On the other hand, any organ not entodermic 

 in origin was thereby branded as illegitimate and excluded from further con- 

 sideration. 



One misinterpretation led to another, till the original theory became so deeply 

 buried that embryologists appeared to forget that the whole superstructure rested 

 on the extremely doubtful assumption that certain stages of vertebrate embryos 

 were to be interpreted in terms of adult jelly-fishes. 



The fact that there was little or no evidence that the archenteron really 

 represents a primitive gut, or that the notochord ever had any other function than 

 it has at present, was persistently ignored. 



As I stated in my earliest paper on this subject, 1889, p. 35 1 : " There is nothing 

 in the embryology of the vertebrates to show to what germ layer the notochord 

 belongs. It is never continuous with functional endoderm ; there is no evidence 

 that it ever exercised, itself, any alimentary functions; or that it is ever con- 

 nected in any way with an alimentary canal." The only thing vertebrate em- 

 bryology tells us about the notochord is that it has its origin, like the axial meso- 

 derm, the nerve cord, and the entoderm, in the common mass of growing cells at 

 the tail end of the embryo. As to the original function of the notochord, or its 

 relation to germ layers, vertebrate morphology has, as yet, had nothing conclusive 

 to say. 



We must, therefore, first identify the representative of the notochord in the 

 invertebrates before we can safely interpret its morphology in vertebrates. This 

 we can do readily enough as soon as we eliminate the misconceptions of the 

 gastrula and annelid theories, for the main facts in the development of the verte- 



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