parts to shew the first appearance of the cartilaginous notochordal sheath 

 which forms the vertebral centra. Stage N. 



Fig. 10. Transverse section through the tail of an embryo of stage P to 

 shew the coexistence of the rib-process and htemal arches in the first few 

 sections behind the point where the latter appear. Zeiss C, ocul. 1. 



Fig. 11. Transverse section through the centre of a caudal vertebra 

 of an embryo somewhat older than Q. It shews (1) the similarity between 

 the arch-tissue and the hyaline tissue of the outer layer of the vertebral 

 centrum, and (2) the separation of the two by the membrana elastica 

 external {m. el) It shews also the differentiation of three layers in the 

 vertebral centrum : vide j). 153. 



1 The slight difference observable between these tv/o tissues in the arrange- 

 ment of their nuclei has been much exaggerated by the engraver. 



