o 



68 LOCV. [Vol. VIII. 



position, in the median line of the embryo, as the latter, but 

 soon after its formation it becomes very much reduced in 

 depth and in longitudinal extent; then it disappears and is 

 replaced at a later stage by the true medullary groove. 



Figs. A', B', C, C", and E' represent five successive stages 

 of embryonic growth and show the way in which this median 

 furrow disappears and gives place to the medullary groove in 

 the neck region of the embryo. I designate the point from 

 which these sections come the neck region, although it is back 

 of the front third of the embryo and considerably behind the 

 expanded cephalic plate. The figures are all (except C") taken 

 as nearly as possible from corresponding regions of the body, 

 and they represent the conditions as fairly as they may be 

 represented by such a limited number of sections. 



In Fig. A', which is the youngest of the group, the median 

 furrow is well developed ; the ectoblast is moulded over the 

 two lateral thickenings of the axial mesoblast and dips down 

 between them, coming in contact with the endoblast it pushes 

 that layer downwards in the median line. The section B' is 

 taken from a slightly older embryo and shows the median 

 furrow relatively more shallow. In Fig. C, which is older 

 than the one just examined, the median furrow has disappeared 

 and the medullary folds {inf.) have made their appearance as 

 lateral expansions of the dorsal wall (medullary plate). Fig. 

 C" shows a section of the same embryo taken about twenty 

 sections behind C . It is introduced in place of a figure D', 

 naturally to be expected in this position, because I have 

 not at present any available section in the right region of 

 embryo D. 



In Fig. E', which is considerably older than any of the 

 foregoing figures, the medullary folds are elevated and in the 

 process of uniting to form the medullary canal. The space 

 enclosed between them is that which corresponds to the 

 medullary groove of other vertebrates, and it is to be carefully 

 noted that this groove is a newly formed one and is not a con- 

 tiniLation of the median fnrrozv which is so ivell marked in 

 Fig. A'. On the contrary, that furrow has disappeared during 

 the intervals of growth, the medullary folds have been produced 



