44 PATTERNS AND PROBLEMS OF DEVELOPMENT 



also more rapid in the median ventral region than laterally and dorsally 

 (Figs. 17 and 18), suggesting a differential from median ventral laterally 

 and dorsally. These relations may be altered by incidental or experimen- 

 tal conditions; anterior or posterior outgrowth may be inhibited differen- 

 tially (pp. 175-76). One example of some interest may be mentioned here. 

 When pieces develop in approximately isotonic or hypertonic concentra- 

 tions of Ringer solution, the cut ends do not contract, as they do in water; 

 the cut end of the gut remains open and usually unites with the ectoderm 

 at both ends of the piece, so that openings into the gut persist at both ends 

 and the regenerating tissue is more or less divided (Figs. 19, 20). When 

 level of section is through the pharyngeal pouch, its wall may unite with 

 the ectoderm and so give rise to a terminal "mouth" with growth of new 

 tissue on each side of it. The inhibition of median regeneration in these 

 cases is evidently an incidental result of failure of the cut surfaces to con- 

 tract in the salt solution and does not represent alteration in gradient re- 

 lations. If the divided new tissue is removed and the piece returned to 

 water, regeneration occurs as usual. Moreover, the wound does not re- 

 main open, as was maintained by J. W. Wilson (1926). 



Rates of development of the head at different body-levels may be com- 

 pared by determining the time between section and appearance of the 

 black pigment of the eyespots. According to a statistical study (Wata- 

 nabe, 1935a) rate of head development, as determined in this way, de- 

 creases from anterior to posterior levels as far as the fission zone and in- 

 creases again in the posterior zooid region of D. dorotocephala. Other 

 planarian species investigated show a similar relation between rate and 

 body-level, except that in those that do not undergo fission the decrease 

 extends to the posterior end of the body or as far posteriorly as head regen- 

 eration occurs." Rate of development of posterior new tissue shows a sim- 

 ilar differential, slight in some species, strongly marked in others, except 

 at extreme anterior levels; heads isolated by section at levels only slightly 

 posterior to the eyespots heal posteriorly without further regeneration; 

 sectioned at levels slightly farther posterior, they regenerate posterior ends 

 incompletely or slowly. Posterior to these levels the minimum length of 

 piece which reconstitutes a complete, normal individual increases posteri- 

 orly to the fission zone in species which undergo fission and is again less 



"Evidence of similar difference of rate has been found in the following: a European 

 form called P. dorotocephala but probably another species (Pourbaix, 193 1) ; a Chinese planarian 

 (Li and Shen, 1934); P. gonocephala (Abeloos, 1930); Dendrocoehim lacteitm (Sivickis, 1931a, 

 1933); Procotylajhiviaiilis (Child, unpublished); Phagocata gracilis (Buchanan, 1933). 



