192 



SCIENCE 



[N. S. Vol. LI. No. 1312 



dermic, since as soon as the so-called "tail- 

 bud " has formed by growth-transformation of 

 the blastoporic lip, differential growth in that 

 region continues to form notochord that has 

 no association with the entoderm whatever. 

 Cerfontaine,^ it may be pointed out, in his 

 classical paper on the early development of 

 Amphioxus, has critically studied the develop- 

 ment of the notochord from the dorsal blasto- 

 poric lip, and accordingly ranks it as an 

 ectodermal structure. 



It is unnecessary to take up here in detail 

 the evidence of the formation of the notochord 

 from the blastoporic lip. There is no reason 

 to consider the development of the chick as 

 exceptional among birds. In mammals, the 

 evidence as it accumulates shows the same 

 mode of origin (from the primitive streak), 

 as exemplified by the recent careful descrip- 

 tion of Huber- for the guinea pig. 



The acceptance of the origin of the noto- 

 chord from the dorsal lip of the blastopore 

 (resp. primitive streak) throughout the verte- 

 brate group (including Amphioxus) leads 

 naturally to the statement that the notochord 

 is to be regarded as ectodermal ia origin. 

 For many years it has seemed to the writer 

 that the conception of a germ-layer should 

 include nore than topographical relation. It 

 is therefore advantageous to consider the blas- 

 toporic lip, primitive streak and so-called 

 "tail bud," undifferentiated material rather 

 than definitive ectoderm, and having within 

 it the " potentialities " of the several struc- 

 tures developed out of it. Its cells would be 

 " totipotent " or at least " pluripotent," if we 

 wish to use these terms. Particularly from 

 the pathological viewpoiut, in the interpreta- 

 tion of teratomata from the persistence of un- 

 differentiated cells of primitive streak or tail- 

 bud origin would this be helpfvil. 



The notochord throughout the vertebrate 

 class shows the marked association with the 

 entoderm, which is of course directly respon- 

 sible for the prevailing view that the noto- 

 chord is an entodermal structure. While in 

 the phylogenetie interpretation of the origin 



1 Cerfontame, P. Arch, de Biol, Vol. 22, 1906. 



2 Huber, G. Karl, 1918, Anat. Record, Vol. 14. 



of the notochord this fact must ultimately be 

 taken fiill account of, ontogenetically, the 

 entoderm is the only one of the three germ- 

 layers which can not be considered as the 

 source of its cells — the one to which it may be 

 referred. Many, as indicated above, from the 

 fact of the superficial location of the forma- 

 tive centers in the blastoporic lip will regard 

 the notochord as ectodermic. One may, as 

 Keibel clearly does,^ consider it unnecessary 

 to refer the notochord to any germ-layer. 

 However, if we must group the notochord in 

 with one of the three fundamental germ- 

 layers, it has seemed to the writer that the 

 notochord must be included among the meso- 

 dermal structures, for the following reasons : 



(1) The mesoderm — or, to make due allowance 

 for other possible sources of mesoderm — that 

 portion of the mesoderm with which the 

 notochord is associated is developed from the 

 blastoporic lip (resp. primitive streak, tail- 

 bud), and is similarly "handled" in develop- 

 ment. When, as in Amphioxus the notochord 

 is at first associated with the entoderm, form- 

 ing temiKirarily part of the roof of the arch- 

 enteron, the mesoderm is similarly associated. 



(2) It attains like the mesoderm an interior 

 (intermediate) position. (3) It is endoskele- 

 tal in its physiologic significance. (4) The 

 notochord in amphibia and reptilia at least 

 gives rise to hyalin cartilage, a tissue of 

 recognized mesodermal characteristic. This 

 seems to be clearly shown by a number of 

 investigators.* Considerations similar to the 

 above led TriepeP to pronounce the notochord 

 a mesodermal structure. 



Were the pathologists to accept the noto- 

 chord as a mesodermal structure rather than 

 entodermal, it may be suggested that the close 

 resemblance of chordomata to myxomata, 

 myxo-chondromata and chondromata, which I 



3 Keibel, Franz, 1900, Anat. Eefte, Vol. X.; 

 Keibel, Fr., 1910 ; Keibel and Mall, Vol. I., Ch. V. 



4 Bruni, A., 1912, Anat. Eefte, Vol. 45. Krauss, 

 Fr., 1909, Arch. f. mikr. Anat., Vol. 73. Pusanow, 

 I., 1913, Anat. Ansieiger, Vol. 44. Sohauinsland, 

 H., 1906, in Hertwig's Sandiuch d. vergel. Entw. 

 ges.. Vol. III., Pt. 2. 



5 Triepel, H., 1914, Anat. Eefte, Vol. 50. 



