42 PRIMITIVE GROOVE IN THE EMBRYO CHICK. 



The epiblast is not thinner where it lines the groove, but the 

 mass of mesoblast below the groove is considerably thinner 

 than at its two sides. This it is which produces the peculiar 

 appearance of the primitive groove when the blastoderm is 

 viewed by transmitted light as a transparent line in the middle 

 of an opaque one. 



This groove, as I said above, is placed at right angles to 

 the long axis of the egg, and nearer the hind end, that is, the 

 narrower end of the pellucid area. It was called " the primitive 

 groove " by the early embryologists, and they supposed that 

 the neural canal arose from the closure of its edges above. 

 It is always easy to distinguish this groove, in transverse sections, 

 by several well-marked characters. In the first place, the 

 epiblast and mesoblast always appear more or less fused together 

 underneath it ; in the second place, the epiblast does not become 

 thinner where it lines the groove ; and in the third place, the 

 mesoblast beneath it never shews any signs of being differentiated 

 into any organ. 



As Dursy has pointed out, there is frequently to be seen 

 in fresh specimens, examined as transparent objects, a narrow 

 opaque line running down the centre of this groove. I do not 

 know what this line is caused by, as there does not appear 

 to be any structural feature visible in sections to which it can 

 correspond. 



From the twelfth to the sixteenth hour the primitive groove 

 grows rapidly, and by the sixteenth hour is both absolutely 

 and considerably longer than it was at the twelfth hour, and 

 also proportionately longer as compared with the length of the 

 pellucid area. 



There is a greater interval between its end and that of the 

 pellucid area in front than behind. 



At about the sixteenth hour, or a little later, a thickening 

 of the mesoblast takes place in front of the primitive groove, 

 forming an opaque streak, which in fresh specimens looks like a 

 continuation from the Anterior extremity of the primitive groove 

 (vide PI. I, fig. 8). From hardened specimens, however, it is 

 easy to see that the connection of this streak with the primitive 

 groove is only an apparent one. Again, it is generally possible 

 to see that in the central line of this streak there is a narrow 



