G C. Davenport. — Primitive Streak. 



PLATE IV. 



All figures on this plate are magnified 73 diameters. The position and direction! 

 of the sections, Figs. 14 to 21, are indicated in Diagram I., page 6, by the correspond-' 

 ing numbers. 



Fig. 14. Sagittal section of embryo ^ (by mistake lettered a) of Fig. 8, slightlyl 

 oblique to the axis of /3. The notochordal canal of /3 {cafi. nfcdA 

 0) has not opened ventrally. The notochordal canal (?) of a (can\ 

 nfcd. a) is cut transversely. Consult text, page 7. 



A section which passes through the left horn of the notochordal open- 

 ing of /3 near its end. 



Section cutting transversely the notochordal canal of a near the mid- 

 dle of its extent. 



Seventh section in front of the anterior lip of the notochordal canal 

 of a. The ectoderm and entoderm of this lip are not completely 

 differentiated. A groove {sul. pr. p.^ lies on the surface of the 

 floor of the canal. 



Cross section of a behind the blastopore. A primitive groove {sul. 

 pr. p.), which is continuous with that of Fig. 17, is present. 



Section showing the primitive streak {sir. pr.), upon which, however, 

 the groove does not extend as far backward as the place of this 

 section. 



Fig- 



Fig. 



18. 



19. 



km 



