l886.] NEW-YORK MICROSCOPICAL SOCIETY. 139 



US again observe the line of contact of the upper and lower parts 

 of the diatom. This line is called the suture. It is a delicate 

 membrane, and extends around the very middle of the outside 

 of the endochrome. It prevents the endochrome from pro- 

 truding, doing in this respect for the endochrome what the in- 

 vesting film does for the dewdrop. It does more ; it feeds, as 

 well as protects. In the complicated system of man, physiolo- 

 gists recognize the play of what is called the osmotic force. 

 After the food is digested, its nutrient principles are given to 

 the blood. They are then distributed by the circulation and 

 transfused through the walls of the veins. So all along the 

 sutural line of the diatom a sort of endosmosis lets the food- 

 bearing water pass into the cell. As already shown, some 

 diatoms are elongated and others circular in shape. On the top 

 of an elongated one we observe along its centre a line or canal 

 with a break at the middle, where the line is interrupted by a 

 vesicular space called a vacuole. The line has also a similar 

 but smaller cavity at each end. These terminal vesicles, it has 

 recently been stated, contain a fine sort of protoplasm which 

 exercises an influence over the organism not unlike that exer- 

 cised over certain of the desmids by the contents of their ter- 

 minal vesicles. Whatever this may mean, the line and the vesi- 

 cles are covered with a non-porous membrane, and the question 

 which concerns us is this : Is the function of the membrane 

 which covers these vacuoles and the median line osmotic, like 

 the sutural membrane ? The discoidal diatoms do not have this 

 central canal. In its place and performing the same function, 

 is a series of cavities around their edges. 



Fifth. How do the diatoms reproduce or continue their 

 species ? I purposely left unmentioned a part of the skeleton 

 known as the band or hoop. After a while, the membrane 

 which we have called the sutural, becomes covered with a sili- 

 ceous deposit which grows into a band or hoop reaching ulti- 

 mately entirely around the diatoms in the sutural space between 

 the two valves. Let us now return and consider the endo- 

 chrome again. The endochrome in the diatom is surrounded 

 by a tissue more highly organized than any other part of the 

 cell contents. This tissue is cellulose. Now, inside of it and 

 close to it is a thin layer of reserve protoplasm which encloses 

 the endochrome and constitutes the formative layer. It is to 



