KOFOID. — LAWS OF CLEAVAGE. 191 



The free larval stage is suppressed and the spiral period of cleav- 

 age persists for a long time, the fundaments of the organs appear- 

 ing comparatively late in development. In the matter of the 

 envelopes of the egg, the three forms present very diverse con- 

 ditions ; yet in all these widely differing forms we find no break 

 in the regular alternation of spirals in successive generations as 

 I have defined them. Previous to the publication of Wilson's 

 paper ('92) on Nereis, which has given such an impetus to the 

 study of cytogeny, the results of a number of investigations 

 appeared dealing more or less fully with the phenomena of 

 spiral cleavage. Neither confirmation nor contradiction of an 

 alternation of spirals implied or expressed in these papers can 

 have the weight of the more recent work on this subject. The 

 examination of the works of Lang ('84), Kowalevsky ('83), Bloch- 

 mann ('81), Rabl ('79), Bobretsky (77), and Fol ('75, '76) has 

 convinced me, however, that the principle of alternating spirals 

 is of wide, if not universal, applicability among the various forms 

 studied by these authors. 



The number of cases in which the law is contradicted is sur- 

 prisingly small. I shall deal with them more fully in my final 

 paper, and for the present shall merely call attention to the nature 

 of the more important contradictions. Some of them rest upon 

 interpretations of the relations of cells which by implication or 

 explicit statement of the author are conjectural, as in Kowalev- 

 sky 's ('83) earlier and Metcalf's ('93) later work upon Chiton. 

 Another kind of contradiction is found in two special cases : 

 (1) in the condition leading to the eight-cell stage of Planor- 

 bis, — a form with sinistral shell, — as described by Rabl ('79, 

 Taf. XXXII. Figs. 9, 10) ; and (2) in the corresponding stage of 

 Janthina — a form with dextral shell (Fischer, '80-'87, p. 77o) — 

 according to Haddon ('82, Plate XXXI. Fig. 6). In both these 

 cases, the spiral of the fourth generation, instead of being a 

 right one, as .in all other cases examined, is apparently a left 

 one. These, however, do not necessarily present exceptions, for 

 an alternation of spirals in successive generations may obtain 

 even here, since the principle of alternation does not necessarily 

 imply that the right spirals should in all forms give rise to the 

 even generations. In other words, we may have in these two 

 forms cases of " reversed cleavage." The decision of this point 

 must be held in abeyance until further investigation can be made 

 on these forms. Still other contradictions belong to a class which 



