34 



J. GRAHAM KERR. 



seems to me of no special weight ; it is merely an additional 

 example of a very common phenomenon, of the fact that 

 hollow organs, formed primitively by involution of a cell- 



Text-fig. 5. — Transverse sections tlirough embryos of various vertebrates to 

 illustrate the formation of mesoblast. — A. Ampliioxus. B. Petro- 

 mjzon. C. Lepido siren. D. Sauropsida. ent. Enteron. m. 

 Mesoderm rudiment, n. Notocliordal rudiment, n.r. Neural rudiment. 



layer, tend, where the cells are burdened with yolk, to 

 arise from a solid rudiment, and to develop their cavity 

 secondarily. 



