CONJUGATE. 105 



original half-cells, which they slightly exceed in size ; after 

 which the two desmids separate along the septum formed 

 across the isthmus. From the above account it will be 

 noticed that in all desmids produced by fission, or division, 

 the two halves of the cell are of different ages, the youngest 

 half being slightly the larger of the two. 



The sexual process takes place by the conjugation of two 

 morphologically or structurally similar individuals in the 

 following manner. In the free species two individuals, 

 lying either parallel or across one another, become involved 

 in a mucilaginous sheath. In each individual the inner 

 layer of the cell-wall protrudes through an opening in the 

 outer wall at the central constricted portion, the two 

 protruded portions become swollen with protoplasm, meet, 

 and after the disappearance of the cell-walls at the point of 

 contact, the two portions of protoplasm combine to form a 

 zygospore, which becomes clothed with a firm, coloured, 

 smooth, or more frequently warted or spinulose cell-wall, 

 and settles down as a resting-spore. The empty half-cells of 

 the two conjugating desmids are pushed oif during the 

 early growth of the zygospore. 



After a period of rest the zygospore germinates and 

 produces at once a new individual, structurally resembling 

 the fully developed form but smaller ; however, by quickly 

 repeated division, the typical size is attained by the method 

 already described. 



In the genus Zygneina and its allies, as Spirogyra, Mouge- 

 otia, etc., the individual consists of a single unbranched row 

 of superposed cells, often of considerable size. In Spirogyra 

 the chlorophyll is arranged in a spiral band, which encloses 

 large starch-grains, that can be rendered very distinct by the 

 application of a solution of iodine, which causes them to 



