THE BLASTOD /;/,' M. 



105 



Its cells are epithelial, and are at first flattened, but subsequently become columnar 

 and larger than those of the epiblast. 



I'l'j. 70.'* Diagrams to show the development of the three layers of the blastoderm ic membrane on transverse 

 section. A. Portion of the ovum with the zona pellucida and the germinal area. B C D E F G. Different stages 

 of development, o. I'mbilical vesicle, a. Amnion. ?. Intestine, p. Peritoneal cavity, bounded by the 

 splanchno-plrural and somato-plenral layers of mesoblast. 1. Vitelline membrane. 2. External blastodermic 

 layer. 3. Middle layer. 4. Internal layer. ;">. Medullary lamina.' and groove. 5'. Medullary canal. 6. Epi- 

 dermic lamiiue. 7. Lateral folds of the amnion. 7'. The same almost in contact. 8. Internal epithelial 

 layer of the amnion. 9. Epidermis of the embryo. 10. Chorda dorsalis. 11. Vertebral laminae. 12. Protover- 

 proper. 13. Muscular laminse. 14. Lateral lamina. 15. Splanchnopleure. 16. Somatopleure. 



-lanchnopleure of the umbilical vesicle. 18. Muscle plate. 19. External layer of the somatopleure 

 20. Internal layer of the same. 21. Mesentery. 22. Splanchnopleure of the intestine." 



*The dotted lines indicate the parts belonging to the internal blastodermic layer: the plain lines, those 

 belonging to the middle: the interrupted lines, those belonging to the external. The embryo has been repre- 

 sented, in this and the following diagram, lying on its back. '1 lie natural position is general!] assumed to be 

 the reverse. 



All the rest of the embrvo is formed from the mesoblast viz. all the vascular 



