78 



occur in the division of tlie nuclei in the Saprolegnieaj. In 

 Peronospora parasitica, however, the nuclei have a definite 

 structure, and the process of division in them is distinctly 

 karyokinetic and closely allied to that which takes place in the 

 nuclei of the higher plants. 



Nuclei of the Hyph.e. 



The nuclei of the hyphse, in the resting stage, are spherical 

 or sliglitly oval bodies (Fig. II. 1, 5.) In those pai'ts of the 

 hyphte which appear to be rapidly growing, the nuclei may 

 become considei-ably elongated and deformed (Fig. IV.) Each nucleus 

 is vesicular, and contains a considerable quantity of chromatin, 

 which is arranged peripherally on its wall. In some nuclei, the 

 chromatin appears to form a continuous ring, while in others 

 it is broken at one or more points, and in others again it pre- 

 sents a somewhat granular appearance. It is, tlierefore, probable 

 that the chromatin body is of a somewhat irregular shape, and 

 may form a coarse network of very thick threads ; or it may be 

 divided into two or more segments. But although indications of 

 the above structure are visible, it is impossible to say with 

 certainty what is the exact structure of the chromatin body in 

 the nucleus in the resting stage. Inside . the chromatin is a 

 substance which does not stain very deeply, and in many cases 

 is not distinctly seen owing to the thickness of the surrounding 

 chromatin. The nucleus appeai-s to be surrounded by a limiting 

 layer or nuclear membrane. 



The division of the nucleus is preceded by a perceptible 

 increase in size, and by considerable changes in the chromatin, 

 leading to the formation, or separation of it into distinct tl^reads. 

 In many cases the chromatin appears to form granules in the 

 nucleus, but tlieir appearance seems to be due to the fact that 

 the nucleus is in such a position as to show only the ends of the 

 threads. The threads appear to be few in number ; they are 

 somewhat elongated, and are arranged irregularly. Tlie nuclear 

 membrane .can be distinctly seen at this Stage; it appears to 

 belong entirely to the nucleus, and it persists, so far as can be 

 seen at present, during the whole of the changes which the 

 nucleus undergoes during its division. The nucleus next elon- 

 gates slightly, and at the same time the threads arrange them- 



