28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



the inner surface of the ectoderm. According to Iwanoff (1928) the 

 mesoderm of the postlarval somites is formed in Polygordins, Aricia, 

 Arenicola, and the OHgochaeta from the mesodermal teloblasts that 

 generate the larval bands of mesoderm, but in the rest of the Poly- 

 chaeta the postlarval mesoderm is proliferated from the ectoderm of 

 the zone of growth. 



The pygidial region posterior to the zone of growth retains its 

 primitive characters throughout the course of development, and is 

 carried continuously backward as the number of somites increases. 

 When the definitive number of somites has been formed, the growing 

 zone loses its distinctive features and becomes indistinguishable as 

 such. Structurally the secondary, or teloblastic, somites are modeled 

 according to the general plan of the primary somites before them ; 

 but, though they may differ in various structural details from the 

 latter, they have one distinctive feature, which is that they alone 

 contain the germ cells. Germinal centers ("gonads") may occur in 

 all the teloblastic segments, but in most of the polychaetes they are 

 limited to a definite part of the body (the epitoke), and in the 

 oligochaetes they are usually restricted to a few segments. 



The ancestral annelids necessarily were reproductive as adults 

 in all their evolutionary stages, but phylogenetic forms recapitulated 

 in ontogeny are generally not reproductive. Hence, it is difficult to 

 study the evolution of the reproductive system from ontogenetic 

 development. The germ cells of the annelids usually are not recog- 

 nizable as such in the larva, and little is known of their embryonic 

 origin. It is claimed by Malaquin (1925), however, that in the 

 serpulid Salmacina dysteri the sex elements first appear as differen- 

 tiated cells in the gastrula, and that later (Malaquin, 1924) these 

 cells become localized immediately before the zone of growth in the 

 posterior segments of the young larva, where they lie ventral to 

 the rectum, and are distinguishable from the surrounding cells by 

 their large, clear, spherical nuclei containing numerous small chro- 

 matic masses. In the oligochaete Pachydrilus, Penners (1930) claims 

 the germ cells arise directly from the mesodermal teloblasts, and are 

 the first cells formed by the latter. The germ cells, as shown also by 

 Penners and Stablein (1930) in Tubificidae, appear prior to the for- 

 mation of the definitive gonad somites, and migrate in the haemocoele 

 to these somites, where they penetrate the mesoderm and finally take 

 their definitive positions in the dissepiments. It seems highly probable, 

 therefore, that the primitive annelids, at a phylogenetic stage before 

 the teloblastic somites were formed, carried the germ cells in the 

 undifferentiated posterior part of the body behind the last primary 



