OF ARTS AND SCIENCES. 33 



first cleavage planes, which he designates as meridian, equatorial, and 

 parallel; but the general significance of these relations has hitherto 

 passed unnoticed. Mark "^ comes very near to this point, when he 

 states that the axis of the ovum is probably homologous throughout 

 the metazoa, and adds that the "maturation spindle " always lies in 

 this axis. We would carry the generalization one step farther, and 

 say it is highly probable that the Jirst cleavage-spindle invariably lies 

 at right angles to the axis of the ovum throughout the metazoa ; ajid 

 that therefore the first cleavage-plane is always a meridian plane, at 

 least in all cases where the first cleavage-spindle is accompanied by 

 cleavage. As the evidence now stands, we cannot affirm that this 

 rule is universal ; but it is so general that the few cases which might 

 be urged as exceptions, can hardly weaken its importance. Among 

 the gastropods we find a very pecidiar departure from this rule, in 

 the case of Massa mtitabilis"'^ and Modiolaria (Crenella) marmorata 

 Forb.^° It is easy to see, however, that the cleavage in these gastro- 

 pods forms no real exception. The equatorial division of the ovum 

 of Nassa, described by Bobretzky as beginning just before the merid- 

 ian cleavage appears, is nothing more nor less than a constriction 

 which simulates a cleavage-plane. That it is not a proper cleavage 

 is shown by its entire behavior, and by the position of the first 

 cleavage-spindle. But what explanation can be given of this con- 

 striction, which after appearing twice vanishes without accomplishing 

 anything? We shall undertake later to show that this phenomenon 

 is only one of many kindred phenomena which may all be ascribed to 

 nuclear influence ; and shall content ourselves here by comparing it 

 with the constriction that comes and goes around the blastodisc of the 

 teleostean ovum, during the early stages of cleavage. It is the same 

 influence which causes the germinal protoplasm to concentrate in the 

 form of a polar disc, and to thicken up until it forms a sort of calotte. 

 The protoplasm gathers up around the two poles of the first cleavage- 

 spindle, as if they were two centres of attraction ; and simultaneously 

 the outward manifestations of the tendency of each half of the disc 

 to assume a spherical condition appear in the form of a meridian 



28 E. L. Mark. " Maturation, Fecundation, and Segmentation of Liniax cam- 

 pestris." Bull. Mus. Comp. Zou!., VI., Part II. No. 12, p. 512, 1881. 



-9 N. Bobretzky. " Studien ii. d. emb. Entw. d. Gastropoden." Arch. f. Mik. 

 Anat., XIII. pp. 98-105, 1877. 



"" Loven. " Bidrag till Kannedomen om Utvecklingen af Mollusca Acephala 

 Lamellibranchiata." Kongl. Vetensk. Akad. Hand!., 1848. (Translation, Arch, 

 f. Naturgeschichte, XV. Jahrg., 1849, p. ."12 ) 



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