324 PATTERNS AND PROBLEMS OF DEVELOPMENT 



the female stage of Stenostomum resembles rhabdocoel species without 

 agamic fission ; heads do not regenerate from levels any considerable dis- 

 tance posterior to the cephaHc ganglia in those species. 



ANNELIDS 



Agamic reproduction by transverse fission and, in some species, by 

 budding appears rather commonly among annelids; zooid chains with 

 morphological development of zooids preceding separation are formed in 

 various polychete and oligochete species, and in the polychete syllids 

 several types of budding appear. In Aeolosoma, Nais, and some other 

 genera of microdrilous oligochetes, each zooid elongates, a new zooid 

 arises in its posterior region, and the length attained by each zooid pre- 

 ceding this division is apparently related, as in Stenostomum, to the stage 

 of development attained by the head region of the zooid. Development 

 of each new head must involve more or less reorganization of the central 

 nervous system posterior to it; consequently, the length of body domi- 

 nated by a developing head region is probably correlated with progress 

 of this reorganization, as suggested above for Stenostomum. 



The position of the fission zone in the oligochete Pristina longiseta can 

 be altered experimentally by nutrition, temperature, and condition of 

 medium (Van Cleave, 1937). With conditions favoring rapid growth of 

 new segments — abundant food, temperature of 2o°-25° C, and fresh cul- 

 ture medium; also with good conditions at 10° C. — new fission zones ap- 

 pear at shorter distances from a head than with poor nutrition at 20°- 

 25° C. or with previously used culture medium and good nutrition at the 

 same temperature. In other words, under conditions permitting rapid 

 growth physiological isolation occurs at shorter distances; under condi- 

 tions unfavorable to growth, at greater distances from a dominant head 

 region. Position of fission zones in various other species is probably simi- 

 larly determined (see, e.g., Eckert, 1927). Among the polychetes, the 

 syllids show various types of budding as well as fissions. Physiological 

 isolation is apparently concerned in these, but experimental data are 

 lacking. 



AGAMIC REPRODUCTIONS IN OTHER ANIMAL GROUPS 



Division in many protozoa is apparently nothing more than cell divi- 

 sion, but in some of the axiate forms there are indications of dominance 

 and physiological isolation. Cytoplasmic reorganization of the body of 

 Paramecium into two zooids begins before nuclear changes are visible pre- 



