No. 2.] THE MEDULLARY GROOVE. 37 1 



During the process of upgrowth of the medullary folds, and 

 also, while the medullary groove is being converted into the 

 medullary canal, the folds in this particular region respond 

 slowest to the new direction of growth and the canal remains 

 widely open above this region for a long time after it has closed 

 behind and in front of it. 



The question naturally arises whether such unusual ventral 

 folding as that shown in Fig. G is not artificially produced, 

 either by the reagents used or by some process of the tech- 

 nique. I have completely satisfied myself on that point. 

 The abrupt folding appears with constant characteristics in 

 all preparations I have ever seen if they are at the right age. 

 I have used a variety of reagents : Davidoff's fluid, picro- 

 sulphuric acid, picro-nitric acid, Fleming's stronger solution, 

 Perenyi fluid, corrosive sublimate and picric acid. All the 

 preparations show this characteristic. In most of my own 

 preparations of these stages the eggs were hardened /// toto 

 before the blastoderm was removed, and as the eggs kept 

 their form, the embryos were not subjected to any consid- 

 erable artificial tension. 



The formation of the medullary folds as described above 

 takes place between Ziegler's ^ Stages C and D, and my studies 

 indicate that it is essential to recognize stages of growth at 

 shorter intervals. If one studies sections of the conventional 

 Stage C, and then follows with sections of the Stage D, as 

 designated by Ziegler, they will completely miss the laterally 

 expanded condition of the medullary folds shown in my figures 

 C and C". My sections of the stage designated D by Ziegler 

 show precisely the same condition of the medullary folds and 

 the medullary groove that he has so well represented in his 

 figures of that stage. 



Balfour 2 first pointed out the ventral curvature of the head- 

 folds of the Elasmobranchs, but he was apparently not ac- 

 quainted with the complete history of the head-folds, and with 

 the method of formation of the medullary folds in the dorsal 



1 I.e. Fig. 4, p. 68 ; Fig. 7, p. 77. 



2 Monograph on the Development of the Elasmobranch Fishes, Plate IX, 

 Fig. 5- 



