44 PRIMITIVE GROOVE IN THE EMBRYO CHICK. 



shew most clearly an adhesion between the epiblast and meso- 

 blast. This fact is both strongly confirmatory of the separate 

 origins of the medullary and primitive grooves, and is also 

 important in itself, as leaving no loophole for supposing that 

 in the region of embryo there is any separation of the cells 

 from the epiblast to form the mesoblast. 



By this time the primitive groove has attained its maximum 

 growth, and from this time begins both absolutely to become 

 smaller, and also gradually to be pushed more and more back- 

 wards by the growth of the medullary groove. 



The specimen figured in PL I, fig. 18, magnified about ten 

 diameters, shews the appearance presented by an embryo of 

 twenty-three hours. The medullary groove (me) has become 

 much wider and deeper than it was in the earlier stage ; the 

 medullary folds (A) are also broader and more conspicuous. 

 The medullary groove widens very much posteriorly, and also 

 the medullary folds separate far apart to enclose the anterior 

 end of the primitive groove (pr). 



All this can easily be seen with a simple microscope, but the 

 sections taken from the specimen figured also fully bear out the 

 interpretations given above, and at the same time shew that 

 the notochord has at this age begun to appear. The sections 

 marked 13 17 pass respectively through the lines with corre- 

 sponding numbers in fig. 18. Section I (fig. 13) passes through 

 the middle of the medullary canal. 



In it the following points are to be noted, (i) That the 

 epiblast becomes very much thinner where it lines the me- 

 dullary canal (me), a feature never found in the epiblast lining 

 the primitive groove. (2) That the mesoblast is very much 

 thickened to form the medullary folds at A, A, while there is 

 no adherence between it and the epiblast, below the primitive 

 groove. (3) The notochord (ch) has begun to be formed, though 

 its separation from the rest of the mesoblast is not as yet very 

 distinct 1 . 



In fig. 14 the medullary groove has become wider and the 

 medullary folds broader, the notochord has also become more 

 expanded: the other features are the same as in section I. In 

 the third section (fig. 15) the notochord is still more expanded; 



1 In the figure the notochord has been made too distinct. 



