136 H. H. NEWMAN. 



of this paper have indicated that all anomalies of this kind are 

 merely induced developmental disturbances and that neither 

 bilateral nor reversed larvae have any more phylogenetic signific- 

 ance than has the occurrence of occasional two-headed fishes or 

 than the sporadic occurrence of sinistral asymmetry in a species 

 of snail typically dextral. 



The Nature and Origin of Asymmetrical Development. The 

 question naturally arises as to why one side of an echinoderm 

 larva should develop structures not duplicated on the other. 

 Two possible explanations suggest themselves: first, that the cells 

 constituting one half of the larval body have a higher rate of 

 metabolism and hence a somewhat more rapid rate of pro- 

 liferation and of differentiation than those of the slower side; 

 second, that the asymmetry is due to some environmental factor 

 that influences one side only or the two sides differently. In sup- 

 port of the first explanation it should be said that the cells 

 destined to form the two bilateral halves of the larva are set apart 

 from a very early period. Driesch (1906) found that bilaterality 

 was definitely established at the eight-cell stage, and presented 

 data that tended to show that the first cleavage plane coincides 

 with the sagittal plane of the larva. In a number of cases he 

 found that when twin gastrulae were formed from the two blasto- 

 meres these twins showed mirror-image symmetry. This obser- 

 vation I have been able to confirm in connection with studies of 

 twinning in Patiria (Newman, '22). Bilateral symmetry then 

 appears to be established at the beginning of cleavage and is 

 probably predetermined in the unsegmented egg. 



More significant still for our problem is the fact that, in a large 

 proportion of twins produced from the physiologically isolated 

 blastomeres of the two-cell stage, the rate of development of the 

 two larvae is markedly different. Almost invariably one of a pair 

 of such twin blastulae or gastrulae within a common vitelline 

 membrane is well in advance of the other. In an earlier stage it 

 can be seen that one twin blastula has fewer cells and a smaller 

 blastoccele than the other. In later stages one gastrula is 

 relatively normal and quite active, while the other is not in- 

 frequently subnormal and shows signs of disintegration. 



These data indicate that the asymmetry which is in later stages 



