EAKLY EMBRYONIC DEVELOPMENTS 1011 



close together, separated by a bright line (primitive streak), but continuous with 

 each other in front. All the transparent zone in front of the primitive line has 

 been designated by Duval as the tergal zone, and we adopt this correct and 

 convenient term. The growth of the primitive line takes place at its posterior 

 extremity, and to this circumstance is due the change in shape of the transparent 

 area, which becomes pyriform. 



It is indispensable to see, on transverse sections, the arrangement of the 

 blastoderm and its layers at the primitive trace. The ectoderm, formed of a 

 layer of cubical cells, is curved in the middle, and its deep face becomes con- 

 founded with the mesoderm. The latter, formed of small angular cells, is placed 

 between the two layers, and enters the substance of the opaque zone. The 

 endoderm, formed of a single layer of fiat cells throughout the extent of the trans- 

 parent zone, suddenly becomes thicker at the opaque zone, by the stratification 

 of its cells still filled with vitelline granules. These marginal thickenings have 

 been named by Kolliker the endodermic ridges ; it is to them that the opaque 

 zone owes its optical characters. 



This transverse section enables us to see, for the first time, the three super- 



Fig. 548. 



TRANSVERSE SECTION OF A BLASTODERM OF THE SAME AGE AS FIG. 547. 



pos., Primitive trace ; ep., epiblast, or ectoderm ; hy., endoderm ; yk., above, the endodermic ridge. 

 The section passes nearly through the middle of the primitive trace. 



posed layers of the germinal disc ; and it exhibits, between the ectoderm and the 

 mesoderm, a curious and characteristic continuity of the primitive trace, which has 

 for a long time exercised the wisdom of embryologists, and given rise to debates 

 now terminated. We will say nothing of the solution arrived at, as it would lead 

 us too far from the scope of this work ; and only refer to the primitive trace, 

 because for a long time it passed for what it was not, and also because there was 

 seen in it a trace of the central nervous system, which only appeared morpho- 

 logically towards the twentieth hour. Destined to retrograde and disappear, 

 the primitive trace is only on the threshold of the embryonal development — an 

 atavic episode, the significance of which cannot be given here, but of which it 

 is necessary to speak, and to point out, in order not to confound it with the 

 origin of the nervous system. 



Article III.— General Direction of Development.— Vertebral Type. 



The ulterior modifications in the three layers of the blastoderm— inflections, 

 invaginations, doublings— tending to the realization of this type, and from the 

 end of the first day, furnish very interesting indications. 



We will study these in a blastoderm from the twentieth to the twenty-fourth 

 hour (Fig. 547). 



