88 



and were then mounted in Canada balsam. The preparations 

 thus obtained, owing to the thinness of the sections, which were 

 on the average only about the so\(3 °^ ^^ inch thick, exhibited 

 the structure of tlie nucleus clearly and distinctly. 



The observations described in this paper, upon the minute 

 structure of the nuclei and sexual organs, have been made almost 

 entirely upon sections obtained in this manner. 



EXPLANATION OF FIGURES. 



Fig. I.— Section showing branches (a) of the mycehum of P. parasitica 

 growing between the cells of the pith of Shepherd's Purse, 

 b. haustoria. Zeiss D. 



Fig. II. — Figures illustrating nuclear division in the mycelium as seen 

 under a low magnifying power. Zeiss D. 



Fig. III. — Nuclei of oogonium in various stages of division, x l,500.i The 

 numbers in the above two figures denote the successive stages 

 of division. 



Fig. IV. — Small piece of the mycelium containing elongated nuclei, x 1,000. 



Fig. V. — Section of a spore on sporangium of P. parasitica, showing the 

 numerous nuclei, n. slightly stained portion of nucleus, c. chro- 

 matin. 



Fig. VI. — Section of an oogonium and antheridium which have just been 

 delimited from the hypha. The nuclei have enlarged slightly, 

 and the chromatin is somewhat broken up. x 1,000. 



Fig. VII. — Section of an oogonium at a little later stage than Fig. VI. ; 

 the nuclei are larger and the thread structure is distinctly 

 visible. Numerous vacuoles v. are present, x 1,000. 



Fig. VIII. — Section of an oogonium with its antheridium. The nuclei are 

 arranged very regularly in the periphery of the oogonium. 

 The central portion of the oogonium contains a quantity of 

 less dense protoplasm ; a, antheridium ; ji, periplasm ; o, 

 protoplasm of the oosphere. x 1,000. 



Fig. IX. — Section through the peripheral portion of an oogonium at the 

 stage shown in Fig. VIII., so as to include a complete layer 

 of the nuclei. The nuclei exhibit a thread structure, many of 

 them in a stage just previous to division. Some very faint 

 conical, slightly stained masses, «, probably of the nature of a 



^ spindle figure are to be seen in connection with many of them ; 



71, nucleus seen from the end ; n^ nucleus seen slightly oblique. 

 X 1500. 



1. Except where otherwise stated, the figures have been drawn by means of tlie 

 camera lucida, und the apochromatic object glasses of Zeiss. 



