EXPLANATION OF PLATES. 



K. I. — Potassium iodide-iodine. 



Ehr. Haem. — Ehrlich 's haematoxylin. 



All of the colored figures represent optical sections and were drawn with 

 the aid of the Abbe camera lucida, Zeiss. Apoc. Horn. Im. Obj. 2.00 mm., 

 Comp. Ocs. 12 and 18. 



PLATE 21. 



Figs. 1 to 5 present a series showing progressive differentiation of the 

 nucleus. Fig. 28 might be looked upon as the first member of the series. 



Fig. 1. Oscillatoria margaritifera. Cross section. Ehr. Haem. The outer 

 narrow layer of cytoplasm is denser and composed of a finer network 

 than the portion of the cytoplasm between it and the nucleus. This 

 is true of all the species figured on this plate. The chromatin is in 

 disconnected masses. 



Fig. 2. Oscillatoria limosa. Cross section. K. I. Ehr. Haem., counter- 

 stained with wasserblau 5 seconds. The chromatin is more abun- 

 dant and more closely connected than in 0. margaritifera. 



Fig. 3. Oscillatoria chalybea. Cross section. K. I. Ehr. Haem., counter- 

 stained with wasserblau 5 seconds. The nucleus in this species is 

 more concentrated toward the center of the cell than in Fig. 2, and 

 the chromatin is in the form of fine threads, somewhat connected. 



Fig. 4. Oscillatoria 0~keni. Cross section. K. I. Ehr. Haem., counter- 

 stained with wasserblau. The nucleus occupies relatively about the 

 same area of the cell as in Fig. 3, though it is more united into 

 thicker masses. 



Fig. 5. Oscillatoria brevis var. Cross section. K. I. Ehr. Haem. The 

 nucleus is more concentrated than in any of the foregoing species. 

 The chromatin is united into a single very coarse net-like mass. 



Fig. 6. Oscillatoria sancta. Cross section. K. I. Aqueous thionin. The nu- 

 cleus is very irregular in outline and the chromatin has a tendency to 

 a radial arrangement. 



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