50 THE FIRST DAY. [CHAP. 



The medullary groove differs in many important particulars from the primi- 

 tive groove. Beneath the primitive groove the mesoblast always fuses more 

 or less with the epiblast ; this is never the case under the medullary groove. 

 Under the primitive groove the mesoblast never shews any signs of differ- 

 entiation into any organ ; under the medullary groove tlie notocbord is formed 

 out of the mesoblast cells. The epiblast lining the bottom of the medullary 

 groove frequently becomes very much thinner than at its sides ; this seems 

 never to be the case with the primitive groove. 



The primitive groove reaches its maximum growth before the appearance 

 of the medullary groove ; and after the appearance of the latter gradually 

 becomes less and less conspicuous, and finally disappears without leaving a 

 trace. A curved remnant of it is to be found at the hind end of the medul- 

 lary canal hetween the 30th and 40th hours, but by the 50th not a trace of it 

 remains. 



By the earlier observers the primitive groove was supposed to become con- 

 verted into the medullary canal. Dursy (Der PrimUivstreif des Hilhnchens) 

 was the first to give a correct account of its disappearance; and the distinction 

 between it and the medullary groove has since been fully recognized by many 

 observers. Goette (Archiv. Micr. Anat. Vol. x. 1873, PP- x 45 — r 99) describes 

 the medullary groove as always appearing to the left of the primitive groove, 

 and having its floor continuous with the left wall of the latter. He states that 

 beneath this left wall the unsymmetrically placed axis-cord is found ; indeed 

 he considers that the notochord is a forward continuation of the axis-cord, and 

 that the latter, as the primitive groove recedes before the medullary groove, 

 becomes continuously converted into the former. 



The primitive groove then is a structure which appears early, and soon 

 disappears without entering directly into the formation of any part of the 

 future animal. Apparently it has no function whatever. We can only sup- 

 pose that it is the rudiment of some ancestral feature. 



6. By the 20th hour the medullary groove or canal, with 

 its medullary folds or laminae dorsales, is fully established. It 

 then presents the appearance, towards the hinder extremity 

 of the embryo, of a shallow groove with sloping diverging 

 walls which embrace between them the remains of the 

 vanishing primitive groove. 



Passing forwards towards what will become the head of 

 the embryo the groove becomes narrower and deeper with 

 steeper walls. On reaching the head-fold (Fig. 11), which 

 continually becomes more and more prominent, the medul- 

 lary folds curve round and meet each other in the middle 

 line, so as to form a somewhat rounded end to the groove. 

 In front therefore the canal does not become lost by the 

 gradual flattening and divergence of its walls as is the case 

 behind, but has a definite termination, the limit being 

 marked by the head-fold. 



In front of the head-fold, quite out of the region of 

 the medullary folds, there is usually another small fold 

 which is the beginning of the amnion (Fig. 11). 



