THE SPIRAL TYPE OF CLEAVAGE. 



239 



become the rule. According to this view the reversed spiral 

 cleavage has arisen from the more common type. 



Our knowledge of the cleavage of reversed forms is very- 

 incomplete as yet. As regards Planorbis, Rabl ('79) shows 

 clearly a reversal of the usual direction of cleavage, and the 

 work of Holmes ('97) affords con- 

 firmation (Fig. 6). Crampton ('94) 

 discovered that the cleavage of 

 Physa is reversed. The case of 

 JantJiina is doubtful. Haddon 

 has figured an apparent left spiral 

 in the third cleavage, huijanthina 

 is, at least usually, dextral. Conk- 

 lin ('97) points out the possible 

 causal relation between reversed 

 cleavage and reversed asymmetry yig. e.—pianorbis. Eight-ceiiitk^bAfi> 



in the adult, but the point I de- ^--P'-^ °f —-d cleavage (after ^,ln^^^).^ 



sire to emphasize is the probable secondary nature ofiithisi 

 reversal, its occurrence as a variation somewhat of the nature 

 of a sport, which, at least in some cases, was inherited! and 

 became the rule. If we accept this view, the occurrencei of 

 sinistral gasteropods must be regarded as without phylogeijnetio 

 significance. : 8£fl 



We find among the gasteropods some species with occasional 

 sinistral individuals, others in which the sinistral individuals ia^e 

 more common, and still others in which all are sinistral. loAi 

 study of the cleavage in species where sinistral forms are ddmi 

 mon is greatly needed. I am inclined to believe, however, that 

 reversed cleavage precedes reversed asymmetry, and furthe'^ 

 that the cause of the reversed asymmetry is the reversal of th)3 

 cleavage and not vice versa ; and, finally, that cases of reversal 

 are secondary modifications, and have arisen after the relation 

 between cleavage and adult asymmetry, of which Crepidula 

 affords an example, had been established. 



Having reached this point in our consideration of the spiral 

 cleavage, it is pertinent to inquire whether we find in this type 

 of cleavage, as it exists at present, any evidence of the conti- 

 nuity of organization, or whether we must admit, as appears to 



