ON THE EMBRYOLOGY OF THE TELEOSTS. 203 



IV. Notociiord axd Neural Cord. 



The origin of the notochord in the Vertebrata has recently been the subject of some 

 discussion, and though we cut no sections we endeavored to make such observations as 

 would throw some light upon its source and the methods of its formation. All of our 

 observations were made upon the living egg, and as we have in a single egg watched con- 

 tinuously every step in the process, and have several times verified all of our results, we 

 feel confident of their accuracy as far as our fish is concerned. To sum up our studies of 

 this point on which we both agree, the notochord arises from the hypoblast, at first as a 

 longitudinal median thickening of that layer and subsequently becomes segmented off and 

 takes its place among the mesoblastic tissues. 



A detailed account of the evidence on which we base this statement will now be given. 

 First an account will be presented of the changes witnessed in a view of the lower surface 

 of the blastoderm and afterward a description of the phenomena observed in optical 

 sections. 



Fig. 24 represents the under surface of the blastoderm at the earliest stage at which 

 the notochord was seen in a flat view. The segmentation cavity (s) posseses the arcuate 

 outline before described while the shaded portion of the figure indicates the extent to 

 which the invagination has extended. The embryonic area (e«) has encroached but slightly 

 upon the cavity. Another sketch shows the same embryonic area on a larger scale. 1 In 

 the median line is seen the notochord extending not quite half way from the margin of 

 the blastoderm to the anterior extremity of the embryonic area. Anteriorly the notochord 

 is well marked and clearly differentiated from the surrounding tissues, while posteriorly 

 this distinctness fades out until at last no line can be drawn separating the chord from the 

 adjacent hypoblast. Anteriorly the cells of the notochord have the same polygonal out- 

 line as have those of the hypoblast, but they are much smaller, indicating that rapid cell 

 division is taking place. As we proceed in our examination, toward the hinder end of the 

 notochord, we find the cells gradually increasing in size and approximating those of the 

 lower germ layer in magnitude until at last no difference can be observed between them. 

 Sometimes a sharp hue may be seen cutting across the extreme end of the notochord and 

 slightly in advance of the margin of the blastoderm and separated from it by a narrow 

 strip of hypoblast, as frequently however this arcuate line was absent, but whether pres- 

 ent or not we were never able to trace the notochord cpuite to the edge of the blastoderm. 

 The margins of the chord at this stage were straight anteriorly but at the hinder end they 

 diverged giving the whole a somewhat spatulate outline. This shape, resemblino- some- 

 what the appearance of a paddle, the blade behind, was retained until a comparatively late 

 stage in the development. 



At this stage, though the notochord was in a great portion of its length clearlv and 

 distinctly outlined, not a trace of cells could be seen extending across the ventral surface 

 of the chord as would have been the case had it been of mesoblastic origin, for then the 

 hypoblast would have extended over the lower surface. 



As the embryo increases in size the notochord becomes longer, apparently growing in 

 both directions but much more rapidly posteriorly, keeping pace with the extension of the 



1 In drawing the plate this figure was inadvertently omitted. intelligible without it. 

 It is hoped that the description is sufficiently clear to be 



