583 



the figures and description given by Kostanecki and Wierzejski 

 as to the spiral asters of Physa are by no means conclusive. I 

 have myself repeatedly looked for such spiral asters in Physa and 

 Planorbis but always in vain. While it is highly probable therefore 

 that the inverse symmetry of the adult is traceable to an inversely 

 symmetrical organization of the unsegmented egg the spiral asters 

 cannot at present be offered as evidence of such inverse organization. 



Rabl has also attempted an explanation of the method by which 

 dextral and sinistral characters may have appeared within the egg. 

 He compares (p. 31) the axial relations of the egg-cells of bilaterata to 

 a double pyramid, the square bases being in apposition and the apices 

 at opposite poles. The chief axis of such a figure runs from apex 

 to apex while the cross axes (Nebenaxen) lie in the plane of the 

 apposed bases and are at right angles to each other and to the 

 chief axis. An egg with such axes he says (p. 32) "would be divided 

 into right and left halves by the first cleavage plane, a half- 

 embryo would develop out of each half and the two half embryos 

 would together form a whole. In the simplest cases in which only a 

 little mesoderm material is contained in the egg cell, or in which the 

 capacity, or potentiality, or the 'stimulus 1 to mesoderm formation is 

 limited to a circumscribed region of the egg cell, the mesoderm 

 material, or the capacity to form mesoderm would be localized in 

 the vicinity of the posterior end of the sagittal cross axis of the lower 

 pyramid. If now the lower pyramid were turned about 90° to the 

 right or left the entire form would undergo torsion around the 

 chief axis. The mesoderm material, or the capacity to form mesoderm 

 would fall entirely into the right or left half and after the formation 

 of the first cleavage furrow the general mesoderm material, or the 

 capacity to form mesoderm would be contained in either the right' or 

 the left blastomere". 



Although such a torsion would explain the fact that in dextral 

 gasteropods the mesoderm arises from the left posterior macromere, 

 while in sinistral forms it comes from the right posterior one, Fig. 7, 

 it would not explain the total inversion of all organs, and it would 

 involve other insurmountable difficulties. If mesoderm Anlagen or 

 primordia may be localized in the egg then those of the ectoderm, endo- 

 derm and of other parts may be also. If the lower pyramid were rotated 

 through 90° the sagittal plane would become the transverse one and 

 vice versa, and not only would the mesoderm be displaced 90° 

 to the right or left but all other parts of the lower pyramid would 

 be equally so. The relations of the lower half of the egg to the upper 



