THE EARLY EMBRYOLOGY OF THE MOUSE 



23 



The neural groove. — It can be seen from Fig. 14B, which is a cross section 

 of the anterior part of an egg cylinder of a 7 14 day embryo, that the ectoderm 

 in the mid-sagittal plane forms a definite trough or truncated V. This 



B. Boht 

 Archenteron Base of notochord 



Ectoderm 



Mesoderm 



Head process 

 entoderm 



C D 



Fig. 14. — Transverse sections of head process. The location of sections is indi- 

 cated on the small key diagrams. A. 7 day i hour embryo. B. 7 day 6 hour embryo. 

 C and D. 7 day 10 hour embryo. Projection drawings (X400). 



trough extends forward in the mid-sagittal plane from the cephalad end of 

 the primitive streak well towards the cephalad limit of the embryonic 

 ectoderm. Developed between the 7 and the 734 day stages (Figs. 14A and 

 B), it is the beginning of the neural groove which later gives rise to the 

 central nervous system. The further development of the neural groove will 



