4 Primitive Streak and Notochordal Canal in Chelonia. 



sections, however, this honeycomb condition is seen to be present 

 to a greater or less degree. 



In drawing sections all outlines were made with the camera 

 lucida. The nuclei were filled in freehand, except in a few in- 

 stances in which it was deemed expedient to use high magnifi- 

 cation. In these instances the outline of every cell, nucleus, and 

 yolk globule was faithfully drawn by means of the camera. Those 

 therefore who disagree with my interpretation of facts may at 

 least have conditions faithfully reproduced upon which to base 

 their own explanations. 



II. Scope of the Paper. 



The scope of this paper is narrow. Not all the questions upon 

 which my sections throw light are discussed. The question con- 

 cerning the homologies of the notochordal invagination is pur- 

 posely avoided for the present. Some observations, however, have 

 been made upon the manner in which the invagination of this 

 canal takes place. 



My sections also throw some light upon the question contro- 

 verted between Will and Mitsukuri concerning the length and 

 width of the notochordal canal. The facts which these sections 

 afford lead me to agree in this matter with Will's theoretical 

 conclusions. 



The much contested question concerning the origin of the 

 mesoderm will not be considered in the present paper. However 

 much Will and Mitsukuri may disagree as to the manner of it, 

 they both testify to the fact that the gastral mesoderm (Rabl, '88, 

 p. 660) arises from the entoderm lateral to the chorda, therefore 

 they both furnish evidence for Hertwig's ccelom theory. Their 

 point of disagreement seems scarcely to warrant the words already 

 spent upon it. Some of my series give me that condition which 

 Mitsukuri ('91, p. 199) considers as positive evidence of coelomic 

 outpocketings, while in other series the " Zwischenplatte " appears 

 solid and flat. 



Concerning the homology between the Rusconian plug of 

 Amphibians and a clump of cells behind the blastopore of Rep- 

 tilia, I find myself in agreement with Robinson and Assheton 

 ('91, p. 477), Keibel ('93, p. 105), and Bonnet ('91, pp. 'jZ, 79). 

 This is the structure which Kupffer ('82) has observed in 



