NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS II 



Msd 



Fig. 5. — Late cleavage stages and mesoderm formation in Annelida and 

 Platyhelminthes. 



A, diagram of posterior pole of annelid blastula showing four yolk-filled 

 "macromeres" of third generation. B, posterior pole of blastula of Arenicola 

 cristata Stimpson after next cleavage forming fourth quartet of "micromeres," 

 showing differentiation of 4d blastomere (adapted from Child, 1900). C, same, 

 after cleavage of 4d into 4(f and 4d' (adapted from Child, 1900). D, blastula 

 of Podarkc ohscura Ehlers, showing mesodermal teloblasts (MsT) derived 

 from blastomeres 4d^ and 4d^ (C) after separation of small endoderm cells 

 (from Treadwell, 1901). E, posteroventral view of 40-hour embryo of Podarke 

 obscura with mouth and anus, showing position of mesoderm bands (Msd) in 

 body (from Treadwell, 1901). F, optical frontal section of embryo of Capitclla 

 capitata Fabr., showing mesodermal teloblasts and rudiments of mesoderm 

 bands (from Eisig, 1899). G, optical section of embryo of DinophUns sp., with 

 mesoderm bands {Msd) extending forward from teloblasts (from Nelson, 

 1904). H, optical section of blastula of Planoccra inquUina Wheeler (Poly- 

 cladia) from right side just after division of 4d, producing 4d^ that will form 

 endoderm, and -/</" that will form mesoblast (from Surface, 1907). I, same, 

 later stage seen from posterior pole, showing mesoblast (Msb*) derived from 

 ^cP, and mesoblast (Msb^) derived from second quartet of ectodermal blastomeres 

 (from Surface, 1907). 



3A-3D, 4A-4D, "macromeres" of third and fourth generations; 4a-4d, "micro- 

 meres" of fourth quartet; An, anus; Bp^r, blastopore; Br, brain; 4d\ ^(f, 

 daughter cells of 4d blastomere ; Ecd, ectoderm ; End, endoderm ; end, endoderm 

 derived from 4d^ and ./(f blastomeres ; Msb', mesoblast derived from second 

 quartet of micromeres ; Msb\ mesoblast derived from 4d cell of fourth quartet ; 

 Msd, mesoderm ; Msnc, mesenchyme ; MsT, mesodermal teloblast ; Mth, mouth. 



