REPRODUCTION AND DEVELOPMENT 



125 



(1959) favours the view that in Glossiphoniidae they are formed 

 from the head ectoderm quite separately from the ventral nerve 

 cord. If this is so, the situation is comparable with that found in 



(c) 



Fig. 77. Later stages in the development of Glossiphonia. 

 (a) dorsal view of an embryo a little older than that shown in 

 75c ; (b), (c) and (d) stages in the appearance of somites and ganglia 

 in lateral view ; (e) dorsal view of a young leech ready to leave 

 the parent ; c, crop diverticulum ; e, eye ; ^1-^3, endodermal macro- 

 meres seen through ectoderm ; gi-gs, three outer rows of cells of 

 germinal bands ; h, head ; n, inner row of cells of germinal band 

 which forms nerve cord ; p, proboscis ; ph, pharynx ; /).^., posterior 

 sucker. After Whitman, 1878. 



polychaetes where the brain and prostomial organs are derived 

 from the apical organ of the trochophore larva and not from the 

 ventral plate. 



The gut is formed from macromeres 4A, 4B, 4C and the fourth 

 quartette of micromeres. At first the endoderm is a syncytium, 

 the nuclei dividing faster than the cytoplasm. Later the nuclei 

 become arranged around the periphery of the yolky mass. The 



