WITH NOTES ON THE WEST INDIAN SPECIES. 



321 



1 doubt whether the enteropneustic stomochord as a whole can be said to correspond 

 to any definite part of the true notocliord. The praeoral extension of the notochord, 

 far beyond the anterior limit of the neural tube in Amphioxus^, is due to a forward 

 growth of the pre-existing notochord ; whereas the praeoral position of the stomochord 

 in the Enteropneusta is due to a forward projection of a portion of the collar-gut or 

 throat. Spengel calls it the " Eicheldarm," but this word, although intended to be 

 indifferent, is apt to mislead, because the stomochord does not belong to the proboscis at 

 all in its primary quality of integral constituent of the gut, but only in its secondary 

 quality of a skeletally metamorphosed derivative of the gut. 



Moreover, whereas the notochord is a uniform, single, indivisible structure, the 

 regional differentiation of the stomochord is, as we have seen, one of its most marked 

 characteristics. It is therefore not sufficient to say that any structure in other forms is 

 comparable to the enteropneustic stomochord, but it must be specified which portion of 

 this structure is referred to. 



The cavity of the stomochord is in an obviously vestigial condition. In the days of 

 its functional activity it must have been a portion of the post-oral gut cavity. Its 

 secondary projection in front of the mouth is a fact which can only be explained at 

 present by assuming a precocious segregation of its primordia, such a segregation being 

 indicated by the fact of its developing from an apparently simple rudiment". 



As I have dealt with this matter in an article which will shortly appear in the 

 Quarterly Journal of Microscopical Science, I can briefly state the conclusions to which I 

 have come. 



1. The " notochord " of Cephalodiscus is related to the vermiform process of the 

 stomochord of Enteropneusta (Spengelidae). This was first suggested by Harmer^ 



2. The pleurochords described by Masterman in the Actinotrocha of the Bay of 

 St Andrews appear to be vestiges of gill-clefts which still persist in Cephalodiscus^. 

 These pleurochords occur in the lophophoral (collar) region of the body. 



3. The pleurochords of Masterman are related to the lateral pouches of the stomo- 

 chord of Enteropneusta. 



4. Thus the lateral pouches of the stomochord may represent the vestiges of 

 a pair of post-oral, but prae-truncal, gill-clefts; gill-clefts having been abolished from 



1 It is this extension of the notochord in front of the cerebral vesicle and neuropore which distinguishes 

 the cephalochordate nature of Amphioxus from the holochordate nature of the Craniota. 



2 The terms rudiment and primordium are not capable of rigid definition and they are often inter- 

 changeable. The rudiment of a new organ is often the vestige of an ancient one, an old vestige becoming 

 transformed into a new rudiment by substitution and change of function. In such cases therefore rudiment 

 is the converse of vestige. Primordium is an independent term, and whereas every primordium is a rudiment, 

 every rudiment is not a primordium. Of course no line of demarcation can be drawn between primordium 

 and rudiment, nor can any be drawn between embryo and larva. The primordium of an organ is to the 

 rudiment of an organ what the embryo of an organism is to the larval organism. 



Mr G. C. Bourne at the meeting of the British Association in Bristol last year, pointed out that 

 the word primordium, in the essential sense in which it is used in the text, originated with William Harvey. 



3 S. F. Harmer, "On the Notochord of Cephalodiscus," Zool. Am. 1897, p. 342. 



4 A. T. Masterman, " On the further anatomy and the budding processes of Cephalodiscus dodecalopbus," 

 Trans. Roy. Soc. Edin., Vol. xxxix. 1898, p. 507. 



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