III. ON THE DISAPPEARANCE OF THE PRIMITIVE GROOVE 

 IN THE EMBRYO CHICK *. 



With Plate I, figs. 68 and 1319. 



THE investigations of Dursy (Der Primitivstreif des Hiihn- 

 chciis, von Dr E. Dursy. Lahr, 1866) on the primitive groove, 

 shewing that it is a temporary structure, and not connected with 

 the development of the neural canal, have in this country either 

 been ignored or rejected. They are, nevertheless, perfectly 

 accurate ; and had Dursy made use of sections to support his 

 statements I do not think they would so long have been denied. 

 In Germany, it is true, Waldeyer has accepted them with a few 

 modifications, but I have never seen them even alluded to in any 

 English work. The observations which I have made corro- 

 borating Dr Dursy may, perhaps, under these circumstances be 

 worth recording. 



After about twelve hours of incubation the pellucid area of 

 a hen's egg has become somewhat oval, with its longer axis 

 at right angles to the long axis of the egg. Rather towards 

 the hinder (narrower) end of this an opaque streak has appeared, 

 with a somewhat lighter line in the centre. A section made at 

 the time shews that the opaque streak is due partly to a thicken- 

 ing of the epiblast, but more especially to a large collection 

 of the rounded mesoblast cells, which along this opaque line 

 form a thick mass between the epiblast and the hypoblast. 

 The mesoblast cells are in contact with both hypoblast and 

 epiblast, and appear to be fused with the latter. The line of 

 junction between them can, however, almost always be made 

 out. 



Soon after the formation of this primitive streak a groove is 

 formed along its central line by a pushing inwards of the epiblast. 



1 From the Quarterly Journal of Microscopical Science, Vol. xm, 1873. 



