<EAD OUTGROV 



NEURAL FOLD 



LLARY GROOVE 



NOTOCHORD 



EU RENTERIC A 



tHTROlD GLAND VENTRAL ACRTAE 



Fig. 230. Sagittal sections of early elasmobranch embryos. (Slightly modified from 

 Scammon. See Chap. 12 in Entwicklungsgeschichte d. Wirbeltiere, by F. Keibel.) (A) 

 Graphic reconstruction from sagittal sections of embryo of 2 mm., seen from left side 

 (condition roughly comparable to stage between fig. 229 A and B). Observe that neural 

 plate is broad and flattened with slight elevation of neural folds. (B) Reconstruction 

 of embryo of 2.7 mm., viewed from left side, showing mesoderm, forming gut, neural 

 tubes, etc. (Consult (C) below.) (C) Same as (B) with mesoderm removed. Observe 

 primitive gut and neural tubes. Note: (B) and (C) are comparable to stage shown in 

 surface view in fig. 229C. (D, E) Same as (B) and (C), embryo 3.5 mm. in length. 

 (This embryo is comparable to fig. 229D.) (F) Same as (D) with mesoderm removed, 

 showing primitive vascular tubes and neural crest cells. 



477 



