FORMATION OF MESODERM 



33 



plate of cells which is the rudiment of the notochord (fig. 66, V. p. 44). In front of 

 the embryonal axis, in most lower mammals, there is an area named the pro-amnion, 

 into which the mesoderm does not spread until a later period, and where therefore 

 the blastoderm is formed merely of ectoderm and entoderm. 



p.st 



FIG. 47. DIAGRAM TO ILLUSTRATE THE SPREAD OF THE MESODERM FROM THE PRIMITIVE 



STREAK, p.st., IN A TYPICAL LOWER MAMMAL. (T. H. Bryce.) 



In a the mesoderm has not yet spread round the entodermic sac, and is undivided ; in b it has 

 completely surrounded the blastocyst and is divided into somatopleuric and splanchnopleuric layers. 

 ect, mes, ent, ectoderm, mesoderm, entoderm. 



The mesoderm sheet now splits on either side into two lamellae parietal and 

 visceral (fig. 47). The parietal layer adheres to the ectoderm, and forms with it the 

 somatopleure ; the visceral becomes associated with the entoderm, and with it 



FIG. 48. DIAGRAM TO ILLUSTRATE A STAGE IN THE 

 DEVELOPMENT OF THE MESODERM LATER THAN 

 IN FIG. 47 b, AND THE FORMATION OF THE 



AMNION, IN A TYPICAL LOWER MAMMAL. (T. H. 



Bryce.) 



">n, am, amnion folds; cce, ccelom; som, 

 somatopleure ; spl, splanchnopleure ; ent , ento- 

 derm of yolk-sac, y.s. 



FIG. 49. DIAGRAM TO SHOW A LATER STAGE 

 IN DEVELOPMENT OF THE AMNION AND 

 YOLK-SAC THAN IN FIG. 48. (T. H. 

 Bryce.) 



a i, amnion, now closed ; cce 1 , intra-embry- 

 onic ccelom; coe-, extra-embryonic cgelom; 

 som, somatopleure ; spl, splanchnopleure ; ent, 

 entoderm of yolk-sac, y.s. 



constitutes the splanchnopleure. The splitting first takes place in the embryonic 



area, and spreads outwards until in certain forms it completely separates the 



blastocyst wall from the entodermic vesicle, now called the yolk-sac. The space 



VOL. i. D 



