PRIMITIVE GROOVE IN THE EMBRYO CHICK. 43 



groove. I do not feel certain that there is no period when this 

 groove may not be present, but its very early appearance has 

 not been recognized either by Dursy or by Waldeyer. More- 

 over, both these authors, as also His, seem to have mistaken 

 the opaque streak spoken of above for the notochord. This, 

 however, is not the case, and the notochord does not make 

 its appearance till somewhat later. The mistake is of very 

 minor importance, and probably arose in Dursy's case from 

 his not sufficiently making use of sections. At about the time 

 the streak in front of the primitive groove makes its appearance 

 a semicircular fold begins to be formed near the anterior ex- 

 tremity of the pellucid area, against which the opaque streak, 

 or as it had, perhaps, better be called, " the medullary streak," 

 ends abruptly. 



This fold is the head fold, and the groove along the me- 

 dullary streak is the medullary groove, which subsequently forms 

 the cavity of the medullary or neural canal. 



Everything which I have described above can without diffi- 

 culty be made out from the examination of fresh and hardened 

 specimens under the simple microscope ; but sections bring out 

 still more clearly these points, and also shew other features 

 which could not have been brought to light without their aid. 

 In PL I, figs. 6 and 7, two sections of an embryo of about 

 eighteen hours are shewn. The first of these passes through the 

 medullary groove, and the second of them through the extreme 

 anterior end of the primitive groove. The points of difference 

 in the two sections are very obvious. 



From fig. 6 it is clear that a groove has already been formed 

 in the medullary streak, a fact which was not obvious in the 

 fresh specimen. In the second place the mesoblast is thickened 

 both under the groove and also more especially in the medullary 

 folds at the sides of the groove ; but shews hardly a sign of the 

 differentiation of the notochord. So that it is clear that the 

 medullary streak is not the notochord, as was thought to be the 

 case by the authors above mentioned. In the third place there 

 is no adhesion between the epiblast and the mesoblast. In all 

 the sections I have cut through the medullary groove I have 

 found this feature to be constant; while (for instance, as in 

 PL I, figs. 7, 9, 17) all sections through the primitive groove 



