Pebbuaet 20, 1920] 



SCIENCE 



191 



group tliat its interpretation — as is of course 

 thoroughly known — has received gi-eat atten- 

 tion, and it plays a part in many of the 

 theories of "the origin of vertebrates." 

 Despite the great theoretical importance at- 

 taching to the origin of the chorda dorsalis or 

 notoehord, we find in the current text-books 

 statements of its origin most conflicting — and 

 as it seems to me unnecessarily so. Of five 

 standard text-books of himian anatomy in the 

 English language, two give the notoehord as 

 entodermal, three as derived from the prim- 

 itive streak. Of five text-books of histology, 

 two describe the notoehord as entodermal, one 

 as ectodermal, while two make no statement; 

 two standard comparative anatomies give the 

 notoehord as entodermal; of seven embryology 

 texts, five state that it is of entodermal origin, 

 although three of these qualify it as an ap- 

 parent origin only, one gives the notoehord as 

 mesodermal, while one states that it may in 

 different vertebrate groups be ectodermal, 

 mesodermal, or entodermal. Three standard 

 text-books of pathology state that the noto- 

 ehord is an endodermal structure. Most text- 

 books of zoology will probably be found to ad- 

 here to the entodermal origin of the noto- 

 ehord. The preponderant statement is thus 

 that the notoehord is an entodermal structure, 

 and since this is the origin in the latest 

 human anatomy and in the latest vertebrate 

 embryology, it is clear that this interpretation 

 is not an old obsolete one held over from edi- 

 tion to edition. 



In the attempt to reconcile the apparent 

 differences of origin of the notoehord or the 

 different interpretations, we have two atti- 

 tudes illustrated: (1) Kellicott in his "Gen- 

 eral Embryology" confessedly accepts an 

 origin from any one of the three germ-layers 

 when he says (p. 358) : The " notoehord may 

 with equal correctness be described as ento- 

 dermal, mesodermal or even ectodermal, in 

 varioiis forms." Kingsley, in his " Compara- 

 tive Anatomy of Vertebrates," who accepts 

 the entodermal origin says, however (p. 18, 

 footnote) : " The statement is made that in 

 some groups the notoehord arises from an- 

 other germ layer than the entoderm, but 



these statements apparently rest on erroneous 

 observations or interpretations. Different 

 origins in different vertebrates would tend to 

 show that what are called notoehord are not 

 homologous." It requires but brief review of 

 the early development of the chick (for ex- 

 ample) to recognize that the notoehord is here 

 developed from the primitive streak and hence 

 not entodermal. Furthermore, the funda- 

 mental plan of the vertebrate body is so con- 

 stant and the occurrence, jjosition, extent and 

 relations of the notoehord so uniform that any 

 suggestion that the notoehord is not homo- 

 logous in the different vertebrate classes must 

 be rejected at once as without evidence. 

 Finally, it would be improbable that such a 

 structure as the notoehord should have funda- 

 mentally different origins in different forms 

 as Kellicott felt forced to asume. 



When the facts of vertebrate development 

 are fully examined, it becomes at once appar- 

 ent that it is unnecessary to assume lack of 

 homology, error in interpretation or real 

 diversity in origin, but that in all vertebrates 

 whose development has been traced — ^from 

 Amphioxus up to man — the notoehord is 

 formed from the dorsal lip of the blastopore 

 or (in higher forms) its equivalent the prim- 

 itive streak. For the preponderance of the 

 view that the notoehord is an entodermal 

 structure perhaps three things are mainly 

 responsible: (a) the prevailing tendency to 

 interpret development as seen in the con- 

 venient transverse plane, with (6) neglect of 

 the concomitant changes in the long axis and 

 without an appreciation of the dorsal lip of the 

 blastopore as the center of differential growth 

 which lays down, along with other structures, 

 the notoehord. (c) The preponderant work 

 done upon the development of the lower 

 vertebrates, particularly Amphioxus and the 

 Amphibia, where, as followed in transection 

 without an accompanying consideration of the 

 growth in the longitudinal planes, it would be 

 unhesitatingly stated that the notoehord was 

 folded off from the entoderm. But even in 

 these forms, it would be only the first, more 

 cephalic, portion, of the notoehord that could 

 be under any interpretation termed ento- 



