Januakt 17, 1908] 



SCIENCE 



97 



plasm of the egg frequently corresponds to 

 the symmetry of the adult. In many bi- 

 lateral animals the fertilized egg is bilater- 

 ally symmetrical. Such bilaterality is 

 found even in the ovarian eggs of insects 

 and cephalopods, while in other cases, such 

 as ascidians and amphibians, it appears 

 immediately after fertilization. In still 

 other groups there is reason to believe that 

 a bilateral organization may be present in 

 the unsegmented egg, even when it may not 

 be directly visible. Thus in most gastero- 

 pods there is no direct evidence of bilater- 

 ality in the unsegmented egg, but in Neri- 

 tina Blochmann observed a group of gran- 

 ules on each side of the animal pole, and 

 in later development these were found in 

 the velar cells of the right and left sides. 

 In all gasteropods the velum arises from 

 similar cells of the early cleavage, and yet 

 the bilateral groups of "Urvelargranula" 

 have been found only in Neritina. Is it 

 not probable that in other gasteropods sim- 

 ilar bilaterally distributed substances are 

 present, although not directly visible ? 



Gasteropods also show a striking correla- 

 tion between the distortion of bilateral 

 symmetry in the spirally coiled body of the 

 adult and the symmetry of cleavage. It is 

 well known that in certain gasteropods the 

 body is coiled to the right, in others to the 

 left, and corresponding to this inversion of 

 the symmetry of the adult, Crampton 

 found that there was an inversion of the 

 symmetry of the cleavage. The cleavage 

 of a half-dozen genera of sinistral gastero- 

 pods is now known and in every instance 

 it is the reverse of that found in dextral 

 gasteropods. Furthermore, this inversion 

 of symmetry may be traced back to the 

 unsegmented egg. Such a case shows con- 

 clusively that not only bilaterality is pres- 

 ent in the cytoplasm of the egg, but that 

 even departures from such symmetry are 

 also present. 



3. In the cleavage of the eggs of many 



animals it has been shown that the position, 

 shape, size and substance of the blasto- 

 meres; the direction, time and quality of 

 cell-division ; the size of the nucleus, centro- 

 somes and chromosomes, are all under cyto- 

 plasmic, rather than nuclear control. That 

 the type of cleavage, its rhythm, rate and 

 direction are determined by the cytoplasm 

 of the egg has been shown by myself in the 

 normal development of gasteropods, and by 

 Boveri and Driesch in the case of echino- 

 derm hybrids ; that the size of the nucleus, 

 centrosomes and chromosomes is dependent 

 upon the volume of the cytoplasm is clearly 

 shown in Crepidula, where, in the large and 

 small blastomeres, these structures are in- 

 variably proportional in size to the volume 

 of the cytoplasm. In the case of echino- 

 derms, on the other hand, Boveri holds that 

 the size of the nucleus is dependent upon 

 the number of chromosomes which it con- 

 tains, and that the size of the cell is con- 

 trolled by the size of the nucleus. The 

 latter is certainly not the case in annelids, 

 mollusks or ascidians. 



4. Another direct correspondence between 

 the cytoplasm of the egg and the struc- 

 ture of the larva may be found in 

 those animals in which particular sub- 

 stances of the egg become localized in 

 definite regions and finally give rise to 

 specific parts or organs of the embryo or 

 larva. Such cases are known among ani- 

 mals belonging to a large number of phyla. 

 Among ascidians the substances which are 

 to enter into the formation of the ectoderm 

 and endoderm, the muscles, mesenchyme, 

 notochord and nervous system are present 

 in the two-cell stage in relatively similar 

 positions and proportions to the corre- 

 sponding organs of the larva. If any of 

 these substances is removed from the egg, 

 the embryo which develops from such an 

 egg lacks the corresponding organ; and 

 conversely, if these substances are forced 

 into abnormal positions in the egg, the 



