106 bulletin: museum of comparative zoology. 



GENERAL SUMMARY. 



A. Observations. 



1. Many divisions take place during the cleaTage of Asplanclina iu 

 which the spindle lies in the shortest axis of the cell, in the direction of 

 greatest pressure, and the ensuing division results in the production of 

 contact surfaces oi greatest area. 



2. In the cleavage of the ectoderm of Asplanchna any cell of any one 

 quadrant cleaves in the same direction as the corresponding cell of the 

 other quadrants, though the forms of the corresponding cells may vary 

 excessively. Conversely, cells of the same form and with similar relations 

 to surrounding cells, but belonging to different layers or series, may 

 divide with spindles in exactly opposite directions. 



3. The entodermal cell follows the same i-hythm and direction of 

 cleavage as the other cells, so long as it I'emains on the exterior and thus 

 corresponds in position with other cells of the eg^. When it becomes 

 enveloped by the other cells, so as to come into different relations with 

 the axis of the embryo, its plan of cleavage changes, showing uo definite 

 relation to that of the ectoderm. 



4. All the cleavages in the ectoderm are to a late period either 

 equatorial or meridional, so that the position of any given spindle is 

 either parallel or perpendicular to that of the preceding spindle. 



5. There is no regular alternation in the direction of spindles. Equa- 

 torial cleavages may follow successively for three or more generations, 

 and the same is true of meridional cleavages. 



6. The position occupied by the two asters after they have passed to 

 opposite sides of the nucleus does not indicate ,the direction of the 

 ensuing spindle. This may occupy the position indicated by the asters, 

 or the definitive position may be gained by a rotation of the asters and 

 nucleus at the passage into the karyokinetic condition. 



7. There is no " regular angle of rotation " (Heidenhain) in a mechani- 

 cal sense, since (a) in cells of different layers, in one case the angle may 

 be zero, in the other case 90 degrees ; and (b) even in cells where the 

 direction of the previous spindle and the direction of the following spindle 

 are the same, the asters may move in an entirely different manner. ]n 

 one cell the rotation may be directly through an angle of 90 degrees, 

 and in a single plane, while in another there may be complex movements 

 and rotation successively in different planes. 



