Page:Popular Science Monthly Volume 79.djvu/579

Rh surrounded by a thickened zone of protoplasm and then each divides twice, thus increasing the number of the zones. All of the protoplasm is utilized in this zone formation, with the exception of a small portion surrounding the original "primary" nucleus, these parts degenerating and disappearing. Preparations of the decalcified individual at this period show hundreds of minute nucleated masses completely filling the shell space (Fig. 6). In life, these emerge in swarms, each in the form of a tiny bi-flagellated swarmer. The swarmers are gametes which

conjugate with similar gametes from another individual (Fig. 8, A). The flagella are thrown off after union, the nuclei unite, and each united pair, as a fertilized cell, or zygote, develops into a new microsphæric individual.

In Polystomella, therefore, fertilization is accomplished, not by union of the parent individuals as in Paramecium, but by coalescence and fusion of minute gametes which contain portions of the specific germ substance in the form of chromidia.