AMCEBOID MOVEMENT OF SPONGE CELLS. 157 



area and therefore cleans up the corresponding portion of the 

 bottom, collects all cells lying in its route and finally forms an 

 aggregate. 



A well formed 24-hour-old aggregate of Microciona has the 

 form of a ball ; its surface is smooth and a thin hyaline membrane 

 can be noticed on its periphery. Under unfavorable conditions 

 the aggregates are irregular in form and fail to form a membrane. 

 They are then unable to undergo futrher transformation and to 

 regenerate into a new sponge. 



DIRECTION OF CELL MOVEMENT. 



There arises a question whether the approaching and the coales- 

 cence of the cells are due to a special kind of chemotropism or 

 " cytotropism " or whether their movement is chaotic and their 

 approach is a matter of a pure accident. If there be a directive 

 force one may expect that the cells will move towards several 

 others which form the centers of the attraction. In other words 

 one ought to be able to detect a definite directive movement. 



The simplest way to study the direction of cell movement is 

 to draw contours of cells at definite intervals and then project all 

 the outlines on one surface and in this way to reconstruct the 

 paths of the cells. Such an investigation was made with a camera 

 lucida and a combination of the Zeiss objective E and eyepiece 

 6 ( X 625). A microaquarium was filled with a dilute suspension 

 of Microciona (i gram of sponge per 200 c.cm. of sea water); a 

 sufficient number of paper sheets were placed on the table at the 

 level of the microscope stage and were pierced at two points. 

 The holes made by the punctures enabled one to put the sheets 

 exactly upon one another after they had been removed from the 

 table. Special precautions were made not to disturb the lower 

 sheets when the upper one was removed. The sketches were 

 made every two minutes and often every minute. The observa- 

 tions lasted from 40 to 190 minutes. After this period the move- 

 ments are so slow that the continuance of the observation was 

 unpracticable. The temperature of the water during the ob- 

 servations varied from 19 to 21 C. The observations were 

 repeated many times with different colonies of Microciona; in 

 all cases the character of the movement was the same, the dif- 



