MUSEUM OF COMPAKATIVE ZOOLOGY. 373 



the two daughter cells (pp. 26, 27). Such was Strasburger's conclusion^ 

 when he undertook its confirmation by the study of cell division on the 

 fresh-water alga, Spirogyra. His results (pp. 34-37, 42-48) may be 

 summarized as follows. 



In the normal quiescent condition of the Spirogyra cell, the nucleus 

 appears fusiform, with its axis perpendicular to the axis of the alga fila- 

 ments.* The firs,t change in a cell about to undergo division is a thick- 

 ening of the nucleus. This is accompanied by a commotion in the 

 enveloping granular protoplasm. The latter stretches out from the ends 

 of the now cylindrical nucleus in the form of suspensory filaments. At 

 length the nucleus has increased its thickness fourfold, and its nucleolus, 

 at first increased in size, has entirely disappeared. Suddenly, after the 

 solution of the nucleolus, the substance of the nucleus exhibits a fila- 

 mentous differentiation, which proceeds from the lateral surfaces toward 

 the equatorial plane. At the same time it becomes condensed in the 

 equatorial plane into a highly refractive plate (Kernplatte). This cen- 

 tral plate exhibits no structural differentiation in the fresh condition, but 

 in alcoholic preparations it shows a continuation of the parallel striations 

 of the lateral halves of the nucleus ; but in the middle the bands are much 

 thicker, and appear like short rods, which are separated by intervals equal 

 to their own thickness. The plate is disk-shaped and reaches, to the pe- 

 riphery of the nuclear mass. The whole striated nuclear structure is 

 surrounded by a hollow cylinder of finely granular protoplasm, leaving 

 exposed only the ends of the nucleus. The insertion of the nuclear fila- 

 ments (Kernfaden), as seen from the end of the cylinder, embraces a cir- 

 cular area, and presents a finely stippled appearance. Simultaneously 

 with these nuclear changes the first evidence of the approaching division 

 appears in the mural protoplasm of the cell. 



By further changes this striate nuclear structure becomes elongated in 

 a direction corresponding to the length of the filaments, with accompa- 

 nying decrease of diameter, and assumes the shape of a cask. Granular 

 protoplasm collects at the ends of the cask-shaped nucleus ; the thick- 

 ness of the nuclear disk becomes increased by the lengthening of its com- 

 ponent rods, each of which now shows a median constriction. In this 

 way the nuclear plate begins to divide into lateral halves (Flatten seg- 

 mente). These plate-segments separate so rapidily that the motion may 

 be directly observed with a magnifying power of 600 diameters. In the 

 separation the halves of the component rods move apart, but drawn-out 

 .filaments' [interzonal filaments] of their substance serve still to unite their 



* It in reality has the form of a biconvex lens. 



