206 KINGSLEY AND CONN 



increase in size of the blastoderm and the growth of the now outlined embryo, the folds 

 anteriorly came closer and closer together and the medullary groove narrower and propor- 

 tionally deeper. It then grew more and more shallow, its decrease in width also continu- 

 ing until at last the groove was entirely obliterated. This groove is shown in fig. 28. 



As before mentioned it was very difficult to obtain surface views of the blastoderm and 

 hence our surface observations on the closing in of the medullary folds have not that 

 detail which we could wish. Were an egg so held that the blastoderm was uppermost, be 

 the pressure never so slight, it almost immediately died and then contracted so that no con- 

 secutive studies could be made. Enough, however, was seen to show that there was none 

 of that infolding and direct formation of a neural canal which is so familiar in the other 

 vertebrates. It was not seen whether the modified closing up took place near the middle 

 and extended both ways or whether from the anterior end backward. From what we 

 know of other forms the former would seem the more probable. 



Fig. 28 shows an optical section through the hinder part of an embryo of the 

 stage shown in fig. 34. Here the medullary groove is shown broad and shallow, the noto- 

 chord has been separated from the hypoblast but has not attained its later quadrate sec- 

 tion but still retains the flattened outline ; the muscle plates on either side have not yet 

 been differentiated from the mesoblast, while the hypoblast extends across immediately 

 beneath the notochord and is not separated from it by any intervening mesoblast. In this 

 optical section it was not possible to make out clearly the cell boundaries although the 

 limits of the various germinal layers were readily made out as figured. 



As to the method in which the neural canal forms, whether in the normal way by an 

 actual enclosing of a tube of epiblast cells, or as maintained by Calberlaby a lumen forming 

 in the epiblast which is pushed down and not> infolded, our observations will not allow us 

 to decide, though I am inclined to believe that the latter is the method, and for this reason. 

 The earliest optical sections of the neural cord do not show any traces of a medullary 

 canal while at a later stage such a canal is found. The same is also true of the brain 

 and the optic lobes. It will readily be seen that, if either of these methods be the true 

 one, the canal so formed is perfectly homologous with the same structure in other verte- 

 brates and hence the actual manner of its formation has not so much importance. It is 

 however interesting to observe that there is the same formation of medullary folds in our 

 fish as are found in other vertebrates, that they arise the same and only differ in the details 

 of the formation of the canal. As was said above these medullary folds exist, but at no 

 time did we see any closer approach to the formation of a closed tube by a longitudinal 

 union of the summits of the neural folds than that shown in fig. 28. Still I am inclined 

 to believe that a large hiatus in our observations may exist here. In a dorsal view 

 of the tail of an embryo with about twenty protovertebrae a well-marked median line 

 was observed which at the posterior extremity slightly broadens out into a groove, just as 

 would be the case did the neural canal form as in other vertebrates. 



Not having witnessed the formation of the neural canal of course nothing definite was 

 seen of a neurenteric canal of the same character as exists in the Elasmobranchs, Batra- 

 chia and birds. In figure 30 which represents the first formation of the neural folds a 

 slight notch is seen at the posterior margin or the blastoderm which afterward 

 became much more marked. This notch arises in the same way and to my mind is 



