423 



Failing to obtain guidance from the other Chordatan groups, we 

 are driven back upon the Ichthyopsida. And the best course to 

 adopt, before any suggestion is made about the phylogeny, is to mar- 

 shal what facts have been gleaned during the study of the skate em- 

 bryos. These are as follows: 1) the hypochord is practically co- 

 extensive with the embryonic gut and with the notochord until the 

 formation of the latter ceases at the posterior end; 2) the hypochord 

 and notochord first appear in the same region, that in which the 

 development of the embryo is most advanced, and therefore the first- 

 formed part; 3) both are derived from chorda-hypoblast ; 4) in their 

 passage out of the hypoblast there is no break in point of time be- 

 tween them, they move together; 5) the hypochord never loses touch 

 with the notochord except when in the last stages of degeneration; 

 6) both develope before any other organs or structures derived from 

 the hypoblast; 7) their histological speciahsation is accelerated and 

 is along the same lines; 8) while the hypochordal cells lie within the 

 hypoblast they may resist pressure when the adjacent hypoblast cells 

 are much flattened, a resistance probably owing to physiological acti- 

 vity; 9) cell-division occurs in the hypochord until shortly after 

 its separation from the hypoblast; 10) it ends its existence within 

 the embryonic period; 11) it disappears long before the notochord 

 reaches its maximum size; 12) it persists after the tail gut, at least 

 just behind the cloaca, if not elsewhere; 13) it is subject to much 

 variation. 



From some of the above facts the conclusion may be drawn that 

 the hypochord is useful to the notochord. But if this is true, the 

 supply of a secretion can only be carried out in a half-hearted way; 

 and this secretion cannot be essential, though it is probably of the 

 same nature as the notochordal secretion. 



We of course want to know what was the primary function of 

 the hypochordal cells. That the hypochord is confined to the Ichthy- 

 opsida is a strong argument in favour of its having originally been 

 an organ sui generis. But it is difficult to believe that two organs 

 — later forming notochord and hypochord — could successively de- 

 velope along the whole dorsal aspect of the gut, for the functions of 

 organs with such an extension would seem to be limited in kind. 

 More probable is it that the whole of the chorda-hypoblast had a 

 single function. If a median part became folded off, perhaps as a 

 water-conducting tube (to form a skeletal structure later), perhaps 

 directly as a skeletal structure, then the residue would remain in touch 

 with the lumen of the gut, at least for a time. It could continue its 



