OF ARTS AND SCIENCES. 41 



impossibility of doing this is demonstrated by Pfliiger's experiments, 

 and herein we find an insuperable objection to his theory. Again, if 

 one cleavage-plaoe has no definite and constant relation to antecedent 

 and subsequent planes, its direction being under the exclusive control 

 of gravitation, it should be possible to turn the second cleavage-plane 

 out of its normal relations with the first cleavage-plane, by turning 

 the ovum after the first cleavage-groove appears. P^xperiments di- 

 rected to this end also gave negative results. The same holds true of 

 the third cleavage, and probably of all subsequent ones. 



A very suggestive fact was brought out in regard to the third cleav- 

 age. If the ovum was turned upside down immediately after the 

 appearance of the second cleavage, the position of the third cleavage 

 was not affected ; but if the inversion of the ovum preceded by an 

 hour or more the be^innins: of the first cleavage, then the third cleav- 

 age appeared in a parallel plane in the uupigmented half of the ovum, 

 and this pole went on segmenting more rapidly than the black pole. 

 The time required to bring about such a remarkable transposition of 

 the third cleavage-plane suggests a corresponding internal transpo- 

 sition of the active protoplasmic matrix of the ovum. If a body con- 

 stituted like the ovum is restrained by artificial means from taking its 

 normal position, a redistribution (Umlagerung) of material must im- 

 mediately set in and continue until the equilibrium is restored. The 

 active basic portion of the ovum, having a lower specific gravity than 

 the passive nutritive elements, would eventually recover its normal 

 position, and thus the virtual axis of the ovum would inevitably right 

 itself in spite of the inability of the ovum to rotate bodily. From 

 this point of view, we may still hold that there is a constant relation 

 between the axis and the cleavage-planes, and that the first two of 

 these are vertical because the axis is vertical, and not because this is 

 the direction of gravitation. 



Cleavage. 



Our observations on the cleavage have been most complete in the 

 case of Ctenolabrus ; fin- here we have followed its entire course from 

 the moment of fecundation onv.ard ; and our observations on the 

 living ovum have been supplemented by a study of sections, and of 

 several complete series of mounted preparations. AVe have followed 

 the cleavage phenomena with somewhat less detail in Pseudorhombus 

 melanof/asler, Ps. oblonr/iis, and Tautoga, but with sufficient care to 

 warrant us in saying that they are in no essential respect different 

 from those observed in the ovum of Ctenolabrus. 



