204 KINGSLEY AND CONN 



blastoderm over the yolk. At the same time other changes may be noticed, in full accord- 

 ance with the idea that the notochord arises from the hypoblast, though they are by no 

 means the sole proof which we have to offer, "as will be seen farther on. Near the 

 anterior end of the chord, the cells of the hypoblast may be seen extending themselves 

 across (on the under surface of) the notochord until these hypoblastic processes from either 

 side meet and close in the chord. The formation of this bridge occupies but a few minutes 

 and is first completed near the anterior portion, from which it progresses at the same time 

 in both directions, reminding one of the closing up of the medullary groove in other ver- 

 tebrates. It has been impossible to obtain any satisfactory sketch of this process as seen 

 in a superficial view. The feature of the egg which has already been mentioned, the 

 greater specific gravity of the germinal portion, readily permitted us to trace these various 

 changes on the under surface of the blastoderm. 



Soon the cord is anteriorly entirely cut off from the hypoblast and closed in ; this process 

 progresses more slowly posteriorly until finally the notochord is wholly separated from its 

 parent layer and is entirely surrounded by mesoblastic tissues, but at what stage the sepa- 

 ration is complete our notes and observations do not show. It is however before the for- 

 mation of the optic lobes and protovertebrae. For a time the cells of the hypoblast can 

 be distinguished by careful focussing extending across the notochord but soon they become 

 so small that it is impossible to recognize them as such with the highest powers which it is 

 possible to use in such investigations carried on upon the living egg. 



This cutting off and closing in of the notochord has been several times witnessed by 

 both of us and it seems as if there were but little chance for errors of observation, but 

 while this -observation from the surface would show that the hypoblast plays a part in the 

 formation of the notochord it does not conclusively prove that it is the whole source of 

 the notochordal cells. So far as these already described observations go all three of the 

 germ layers may play a part in its formation and there might be some truth in Rudwaner's 

 statement ('76 p. 161) that the notochord arises from the epiblast. We have however 

 other evidence which proves to us conclusively that the hypoblast alone gives rise 

 to the notochord. 



Returning to the invagination described above ; its later stages and its connection with 

 the formation of the chorda dorsalis may now be described. These various steps have 

 been constantly and consecutively watched by us both in several eggs, not only of the 

 dinner but in several other forms, as well as by Mr. Van Vleck in the eggs of Merlu- 

 cius and there exists in the minds of us three not the slightest doubt of their general 

 accuracy. 



In optical sections it was seen that the invaginated hypoblast was but a single layer of 

 cells in thickness and at all times was separated from the overlying mesoblast by a well 

 defined line. An optical section, transverse to the longitudinal axis of the embryo, was 

 closely watched and at first the hypoblast was but a single layer of cells deep and every- 

 where of uniform thickness. Soon a thickening was seen in the median line of the 

 hypoblast, extending slightly into the mesoblast and also into the underlying yolk or 

 intermediary layer. This thickening was clearly a part of the hyjioblast and the meso- 

 blast was in no way concerned in its composition, the line of demarcation between the 

 two layers being as well defined as before. Gradually a sharp line appeared cutting the 



