No. I.] POLYCHOERUS CAUDATUS. 89 



nucleus. In Figs. 16 and 22 the length has evidently lessened 

 considerably. It must either be that the substance of the 

 spindle has undergone a condensation, or else that certain 

 material has been dissolved into the cytoplasm. There is, how- 

 ever, no evidence as to how either of these changes may have 

 been brought about. I have endeavored to determine the rela- 

 tion which the axis of the first maturation spindle, when first 

 formed, bears to the axis of the first segmentation spindle, but 

 with no satisfactory result. Fig. 26 is a diagram showing the 

 dumb-bell-shaped form of the first segmentation spindle ; AA 

 the plane of the first cleavage ; BB the plane of the second 

 cleavage ; P the polar bodies near the surface of the ovum. 

 The plane AA divides the ovum into two equal macromeres. 

 The plane BB cuts off two small cells which lie on the surface 

 of the macromeres. The polar bodies always lie near the sur- 

 face of one of the macromeres and near a point where AA and 

 BB intersect. It is evident that if the first maturation spindle 

 swung through an arc of 180° to break free from the nucleus 

 remnant, and then traveled in a straight line to the egg surface, 

 a line drawn through this amphiaster in anaphase when in con- 

 tact with the egg membrane as shown in Figs. 16 and 21, and 

 through the center of the ovum where the nucleus was, would 

 give the axis on which the amphiaster was formed. Since, 

 however, these facts cannot be accurately ascertained, except 

 by studying whole transparent ova, the relationship of these 

 axes still remains uncertain. 



As the amphiaster moves towards its destination a very 

 distinct and beautiful cytoplasmic network is formed, extending 

 out from the asters. This could of course only be accomplished 

 by the continual breaking down and building up of the cyto- 

 plasmic material ; for while the whole structure is in motion 

 there is at no time a distortion of the network (Fig. 22), such 

 as would result by the movement of the structure through the 

 ovum, — unless, indeed, the cytoplasm were very fluid and the 

 network a rigid structure. That the amphiaster is much more 

 rigid than the surrounding cytoplasm is shown by two instruc- 

 tive preparations which were the result of accident. Ova con- 

 taining amphiasters in the stage now under discussion were 



