BY WILLIAM A. HASWELL, M.A., D.SC. 599 



Fig. 12. — Section of the same a little further back, with complete hypoblast 

 layer (hy. ) 



Fig. 13. — Section of the same showing secondary cleft in primitive streak. 

 pr. primitive groove, ms. mesoblast. ep. epiblast. «. x-emark- 

 able granular cell in hypoblast below the cleft. 



Plate XIII. 



Fig. 14. — Section of the same at the extreme posterior end of the primitive 

 streak. 



Fig. 15. — Transvei'se section through the head-swelling of embryo of 78 

 hours (fig. 3). 7ng. anterior part of medullary groove, ep. 

 epiblast. ms. mesoblast. fy. commencing fore-gut. 



Fig. 16. — Section of the same a little further back ; letters as before. 



Fig. 17. — Central part of the same section more highly magnified. 



Fig. 18. — Section of the same blastoderm passing through the hinder part 

 of the medullary plate, with the rudimentary notochord (n. ch. ) 

 separated from the lateral plates of mesoblast (ms.). 



Plate XIV. 



Fig. 19. -Section showing the transition from the medullary plate to the 

 region of the primitive streak : the notochord (n.ch.J passing 

 into the axial plate. 



Fig. 20. — Section a little further back behind the termination of the noto- 

 chord. 



ig. 21. — Section of 115 hours' embryo (fig. 8) just in front of the neuren- 

 teric canal, showing the continuity of the hypoblast (hy.) with 

 the notochord (n.ch.) at this point, m. medullary canal. 



Fig. 22. — Section of the same a little further back, passing through the 

 neurenteric canal. 



Fig. 23. — Section of the same embryo a little behind the neurenteric canal. 

 Letters as before. 



