5S6 PATTERNS AND PROBLEMS OF DEVELOPMENT 



Removal of parts of the egg between fertilization and first cleavage 

 shows increasing disturbance of cleavage pattern the later the removal, 

 but essentially normal larvae may develop. In isolated 1/2 and 1/4 blas- 

 tomeres the cleavage pattern is a half or fourth of the whole, except as 

 shifts in cell position occur; but normal larvae may still result (Wilson). 

 Developmental effects of removal of parts of one or both blastomeres dur- 

 ing or after first cleavage are slight (Yatsu). The apical quartet of the 

 eight-cell stage gives rise to larvae with apical flagellum but without 

 enteron; the basal quartet, to larvae with enteron but no apical organ 

 (Zeleny) . Apical and basal parts of blastulae isolated by section show much 

 the same differences (Wilson). The three experimenters conclude that 

 there is progressive apicobasal localization of formative materials during 

 early cleavages. More recent experiments agree in general with these re- 

 sults. Isolated blastomeres of the two-cell stage may form complete, sym- 

 metrical larvae, an extensive reconstitution. The four apical or the four 

 basal cells of the eight-cell stage, when isolated, give rise to apical and 

 basal partial forms; and isolated rings of cells an^, an^, veg„ and veg., 

 of the sixteen-cell stage, ^ and various combinations of these rings, devel- 

 op as if the other blastomeres were present (Horstadius, 19376). Nothing 

 is known at present concerning gradients in the nemertean egg; but the 

 unfertiUzed egg evidently possess a polarity, and its high reconstitutional 

 capacity suggests that this polarity is a predominantly quantitative dif- 

 ferential at that stage but undergoes progressive regional differentiation in 

 early development. Certainly neither the egg nor early cleavage stages 

 are mosaics. 



DUPLICATIONS AND OTHER RECONSTITUTIONS IN Tubifcx EMBRYOS 



It was noted earher (p. 537) that development of complete or partial 

 twins from single eggs is not infrequent in certain oligochetes under sup- 

 posedly natural conditions. Partial duplications may involve either the 

 anterior or the posterior end or both, but in Tubifex these duplications orig- 

 inate in different ways (Penners, 1924a, b). Simple duplications of the 

 anterior end result from failure of the germ bands of the two sides to 

 come together anteriorly; the bands of each side give rise to an anterior 

 end, symmetrical as regards body wall, nervous system, and entoderm 

 but with only one dorsal and one ventral series of setae and one nephrid- 

 ium in each segment, seta-sacs and nephridia of the apposed sides not 

 developing. Penners suggests that these forms result from action of some 



s The designations aiir veg2 are the same as used for the sea-urchin embryo (p. 438). 



