74 



FECUNDATION ; NON-MOTILE ISOGAMETES. 



in general two pyrenoids, undergoes a rejuvenescence and finally 

 divides. Prior to this cell-division, however, two successive mitotic 

 divisions of the nucleus 1 take place (Fig. 22, A to E). After the first 

 mitosis the daughter nuclei generally move apart toward the ends of 

 the cell whither the pyrenoids also wander (Fig. 22, B). Soon the 

 second mitosis takes place, when four nuclei similar in appearance are 

 present in the protoplast, which may, as yet, show no sign of division 

 (Fig. 22, D). With further progress the protoplast in each individual 

 becomes constricted near the middle and finally divides, two daughter 

 nuclei passing into each daughter cell, which contains one or some- 



FIG. 23. Conjugation of gametes in Rhopalodia gibba. (After Klebahn.) 



F, conjugating pair seen from valve side ; protoplast of each has divided into two daughter cells or 



gametes ; each gamete contains, besides the large pyrenoid, a large and a small nucleus. 



G, cytoplasmic fusion of the two pairs of gametes has taken place; the small nuclei are scarcely recog- 



nizable. 



H, a later stage ; the small nuclei have entirely disappeared, while the two functional nuclei in each 

 zygote, which has now changed from a dumbbell to an elongated form, have come nearer together. 



times two pyrenoids and a chromatophore (Fig. 22, E). A marked 

 change is now manifested in the nuclei. Of the two nuclei in each 

 daughter cell, one increases in size while the other diminishes, becom- 

 ing dense and contracted (Fig. 22, D, E). The next step in the pro- 

 cess is the conjugation of the daughter cells of one individual with 

 those of the opposite one by means of protuberances sent out from 

 the respective cells (Fig. 23, F, G). The large nucleus of each 



1 For details of mitosis see the original paper of Klebahn, '96. 



