228 



ZOOLOGY. 



FIG. 104. Diagrams of stages in the metamorphosis of Polygordius, a primitive 

 annelid. Ectoderm throughout is represented as nucleated without cell boundaries; the 

 entoderm has the cell-boundaries shown, and the mesoderm is diagonally shaded. A, 

 gastrula; B, same with blastopore closed; C and D represent formation of stomodaeum 

 and proctodaeum from ectoderm; E, Trochosphere stage showing formation of segments 

 in the posterior portion; F, adult (sagittal); G, adult (transverse), a, archenteron; 

 bp., blastopore; br, brain; c, ccelom; d, dorsal; di, dissepiments; m, mesenteron; pr., 

 proctodaeum; s.c., segmentation cavity; st, stomodaeum; v.n., ventral nerve chain; s, 

 zone of formation of nerve segments. After Fraipont. 



Questions on the figure. Trace the behavior of ectoderm and ento- 

 derm in these figures and determine what structures each seems to give 

 rise to. What is a Trochosphere? Distinguish between somatic (body) 

 and splanchnic mesoderm. (See 56.) 



ment in a free state. Segmentation in Annulata is complete 

 and usually unequal, giving rise at the eight-celled stage to 

 four micromeres and four macromeres (Fig. 103). The 

 micromeres produce the ectoderm; directly or indirectly the 

 macromeres give rise to the entoderm. Early in the cleavage 

 " primitive mesoblasts " cells which produce the mesodermal 

 structures, are separated from the macromeres. A gastrula 



