. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. 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 appar
. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. 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 individualsia^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
Size: 1626px × 1537px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, bookpublishe, booksubjectbiology
