560 MAYOR. 



Two nuclei are found in the myxosporidium in excess of the twelve 

 concerned in the formation of the two sporoblasts (PI. II, Figs. 28 and 

 32, where in each case only one of the two nuclei is shown. These 

 are the trophonuclei of the earlier stages. They gradually degenerate 

 as spore formation advances, but often are still to be seen when the 

 myxosporidium contains two almost completely developed spores. 

 Since these nuclei degenerate and no other nuclei are found in the 

 myxosporidium outside the sporoblasts, the myxosporidium must cease 

 to exist at the end of spore-formation. These nuclei are possibly to 

 be homologized with the residual nuclei of the polysporea. 



The two sporoblasts are formed in juxtaposition, and remain to- 

 gether throughout their development into spores. In the parasite of 

 U. chuss the fully formed spores, even after there is little trace of the 

 myxosporidium left, are nearly always found in pairs, the members 

 of which are in a definite relative position with regard to each other 

 (PI. II, F'ig. 39). This is not as evident in the case of the parasite of 

 P. americanus. Their position would suggest a close relation between 

 the sporoblasts and perhaps the presence of some structure enclosing 

 them both. However, no membrane or other limiting structure could 

 be seen surrounding the sporoblasts. The pair of sporoblasts may lie 

 anywhere in the myxosporidium, with their long axes parallel to, or 

 making any angle with the long axis of the myxosporidium. The 

 absence here of any structure suggesting a pansporoblast — Pans- 

 poroblast, Gurley ('94); Sporoblast erster Ordnung, Butschli ('81); 

 Sphere primitive, Thelohan ('95) — agrees with the conditions de- 

 scribed by Awerinzew ('09) for Ceratomyxa drepanopsettae Awer.; 

 but in the case of his species the sporoblasts seem not to preserve any 

 definite position in relation to each other (cf. Awerinzew : 09, Taf. 7, 

 Fig. 26, 27, 30). Neither Dofiein ('98) nor Thelohan ('95), both of 

 whom studied the formation of the sporoblasts in Ceratomyxa, men- 

 tion the presence of a pansporoblast, and their figures show no such 

 structure. The absence of a pansporoblast from Zschokkella hildae 

 Auer. is also mentioned by Auerbach (:09), and from Chloromyxum 

 cristatum Leger, by Leger ( : 06) ; but in these cases, the sporoblasts 

 are formed singly. 



The pansporoblast of the Polysporea contains, in the case of forms 

 with two polar capsules, fourteen nuclei. When these nuclei, with 

 the differentiated protoplasm surrounding each of them, become asso- 

 ciated in groups of six to form the two sporoblasts, two of the four- 

 teen nuclei remain in the pansporoblast as "Restkerne," or residual 

 nuclei (cf . Schroder, : 07, Taf. 15, Fig. 32, and Keysselitz, : 08, Taf. 13, 



