No. 427.] THE EGG OF BUFO LENTIGINOSUS. 545 
as a distinct rounded structure, the notochord (Eig: 22, NJ. 
For only a short distance at this stage of development is the 
notochord entirely cut off from the mesoderm ; beyond there 



DDE 
1700 
XU A 






Fic. 20. — Part 
f l section through an egg at the stage of Fig. 7, showing the mid-d 
region of the embryo just in front of the blastopore. ÆC., ectoderm; ÆN., endoderm; 
JI., mesoderm, 
is the same thickening of the mid-dorsal portion of the meso- 
derm sheet as seen in Fig. 21. Inthe head region of the embryo 
the mesoderm is thinned out to a layer one or two cells deep 
Which shows absolutely no thickening in the middle and appears 
very much as in Fig. 19. After the blastopore has closed, the 
notochord becomes extended forward in the head region and 
backward in the region of the tail. 
Schultze, Goette, Lwoff, Brauer, Schwink, and Morgan also 
believe that the notochord is derived from the mesoderm ; while 





«sgg = 
SK FQ 
Fic. 21. — Part of a section from the same egg as Fig. 1 slightly 
EC., ectoderm; ÆN., endoderm; M., mesoderm. 


ant to it. 
other investigators consider that in the amphibians, as in other 
vertebrates, the notochord is endodermal in origin. Although 
Field (5) believes that in Bufo vulgaris the notochord arises 

