282 



BIOLOGY 



FIG. 132. DIAGRAM REPRESENTING THE DEVELOPMENT OF THE PROG 



A, eight stages of the segmentation of the egg; B, section of the egg showing the beginning 

 of the differentiation of ectoderm from endoderm; C, sections at a later stage, showing the 

 growth of the ectoderm over the endoderm; D, section after the germ layers are formed; 

 c, ectoderm; en, endoderm; mes, mesoderm; E, surface view of a young embryo showing 

 two branchial slits, brc; F, surface view of an older embryo; G, diagrammatic, longitudinal 

 sections of the stage F ; H, a later stage. In these diagrams the ectoderm is in black, meso- 

 derm, with dotted shading, and endoderm without shading. 



br, the brain; I, liver; nc, notocord; sp, spinal cord. 



ht, heart; n, nervous system; s, sexual duct; 



(Various authors.) 



