BLASTODERMIC LAYERS. 359 



more granular and darker than the rest of the blastoderm. 

 The elongated line thus formed and surrounded by the area 

 pellucida is called the primitive trace. 



Almost all writers have regarded the primitive trace as 

 the seat of the development of the spinal column of the 

 embryon ; but some doubt has been thrown upon this point 

 by the observations of Dursy, in 1866, quoted and recently 

 confirmed by Balfour. According to these observations, 

 which were made upon the chick, the primitive trace, or 

 primitive groove, is a temporary structure, and has nothing 

 to do with the development of the neural canal. After this 

 groove is formed, Balfour states that there appears in front 

 of, but not continuous with it, a new fold and a groove lead- 

 ing from it. This is the " head-fold," and the groove is the 

 true medullary groove, which is subsequently developed into 

 the neural canal. 1 If we adopt this view, and the difference 

 is not very important, we simply substitute the new trace, 

 the seat of the development of the neural canal, for the 

 original primitive trace, which is temporary. It is prob- 

 able that embryologists have heretofore noted the so-called 

 primitive trace and studied subsequently the development of 

 the true medullary groove, supposing that they were iden- 

 tical structures in different stages of formation, not observ- 

 ing that the first trace disappears. 



Blastodermic Layers. Shortly after the appearance of 

 the primitive trace, the blastodermic cells, which are at first 

 arranged so as to form a single membrane, separate into 

 layers. These layers have been differently described by 

 various observers, and there is some uncertainty with re- 



1 BALFOUR, On the Disappearance of the Primitive Groove in the Embryo 

 Chick. Quarterly Journal of Microscopical Science, London, 1873, New Series, 

 No. li., p. 276, et seq. 



The mutual relations of the primitive trace and the medullary groove in the 

 chick are figured in the elaborate work of His. (Untersuchungen uber die erste 

 Anlage des Wirbelthierleibes. Die erste Entwicklung des Huhnchens im Ji, Leip- 

 zig, 1868, Taf. xii., Fig. 14.) 



