540 PATTERNS AND PROBLEMS OF DEVELOPMENT 



to environmental factors, for the first duplication is right and left with 

 respect to the parent, on the two sides of the primary embryonic vesicle 

 toward the openings of the Fallopian tubes, and the second duplication is 

 in definite relation to the first. 



Newman (1923) holds that this polyembryony is associated with occur- 

 rence of a quiescent period in development, during which the original 

 developmental pattern is, to a large extent, obliterated; and with renewal 

 of developmental activity the reaction to, and the determination of, a 

 new pattern by environmental differentials within the uterus results. 

 Even after this, however, the integrating factor is not adequate to pre- 

 vent a second physiological isolation of parts ; consequently, four embryos 

 are formed. The entire sequence of reconstitutions in D. hyhridus has not 

 been followed, but the irregular arrangement and variable number of em- 

 bryos in that species indicate that some of the embryonic primordia bud 

 or divide more than others. There seems to be no doubt that this type of 

 polyembryony involves repeated obliteration and determination of de- 

 velopmental patterns, and its occurrence in the mammals suggests that 

 under proper conditions pattern in various other eggs may not be as stably 

 determined as commonly assumed. 



FUSIONS OF EMBRYONIC AND LARVAL INDIVIDUALS 



Larvae of the sponge Lissodendoryx, merely brought into contact at 

 certain stages, fuse readily into masses consisting of indefinite numbers of 

 larvae in which all evidences of individual form and polarity disappear. 

 These masses, in contact with a solid substratum, may metamorphose 

 into perfect sponges (H. V. Wilson, 1907). Evidently there is complete 

 obliteration of the original larval patterns in these masses, and the polarity 

 of the resulting sponge is determined anew by an environmental differen- 

 tial, probably the differential arising between free surface and surface in 

 contact with the substrate, since development of the osculum or oscula 

 is on the free surface, as in aggregates of dissociated sponge cells (p. 418). 

 Like the cell aggregates, these fusion masses would be interesting material 

 for other experiments on determination of pattern. 



Normal planulae from fused blastulae of the medusa Milrocoma have 

 been described, but nothing is known concerning possible changes in pat- 

 tern. Fused cell masses resulting from cleavage in calcium-free sea water 

 of eggs of the nemertean Cerehratidus may, after return to normal sea 

 water, develop into giant pilidium larvae with multiple organs (Yatsu, 

 1910c). Apparently in these the original pattern is not completely oblit- 



