414 ZOOLOGY. 



which are the gametes, that is to say the cells which con- 

 jugate. In each gametocyte about to segment the nucleus 

 undergoes the following changes. The karyosomes break 

 up and become partially dissolved in the nuclear sap, and 

 a number of grains or short filaments of chromatin appear, 

 which are believed to be derived from the karyosomes. 

 These chromosomes are collected in a group and form the 

 segmentation nucleus. The nuclear membrane disappears, 

 and the segmentation nucleus divides by mitosis (Fig. 

 3, E). The remains of the karyosomes are left free in the 

 cytoplasm and are gradually absorbed. The numerous 

 nuclei formed by division pass to the surface of the game- 

 tocyte and its cytoplasm divides to form cells around them. 

 These are the gametes, and the central cytoplasm of the 

 gametocyte is left as a cystal residuum (Fig. 205, r) . 



3. Conjugation and Spore-formation. The gametes 

 exhibit for a time active movements, and then conjugate 

 in pairs, forming the zygotes, or sporoblasts, in which the 

 two nuclei of the two united gametes fuse together (Fig. 

 3, o). The zygote becomes a spore: it assumes an oval 

 form, and secretes a tough membrane or sporocyst. 



The spores have a characteristic fusiform shape and are 

 very conspicuous in preparations of the seminal vesicles. 

 From their resemblance to a diatom called Navicella, they 

 have been known as pseudo-navicellae. 



4. Formation of Sporozoites. A new process of division 

 takes place in the spore within the sporocyst, the nucleus 

 dividing three times, so that eight nuclei are formed. This 

 division is amitotic. The cytoplasm segments to corre- 

 spond with the nuclei, each segment being called a sporo- 

 zoite, and as before a central residuum is left unused. 

 The sporozoite has been sometimes called a falciform body. 



5. Mode of Infection. Nothing is known with cer- 

 tainty of the means by which the spores are transferred to 

 other earthworms, although as a rule every earthworm 

 examined contains the parasite. It might be supposed that 

 the spores would be transferred from one host to another 

 in the process of copulation, when the spermatozoa and 

 seminal fluid are conveyed from the seminal vesicles of one 



