54 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



to form what may appear to be a chromatic reticulum, its attachment to 

 the central body is still conspicuous and indicates that each strand main- 

 tains its separate and individual connection with the nuclear pole. In 

 favorable preparations close analysis of the resting stages show consid- 

 erable evidence that the eight chromosomes are still to be differentiated 

 as constituting the main strands of the reticulum, though they may be 

 quite irregular in their outline and connected by anastomosing fibrillas. 

 The central bodies are thus seen to be permanent structures of the cell 

 during both the dividing and resting stages of nuclear development. 



In the processes of nuclear fusion in Phyllactinia the permanence 

 of the central body is also evident. In the vegetative nuclear fusions 

 in the young ascus each nucleus has a conspicuous polar structure and 

 central body at the time when the nuclear membranes break down and 

 the nuclear cavities combine. The fusion nucleus thus formed has for a 

 time two centers and two independent systems of strands of chromatin. 

 These, however, gradually approach and combine into a single centered 

 system with one central body. The centers fuse and the chromatin 

 strands combine, so that the fusion nucleus has one central body and 

 the same apparent number of strands of chromatin as each of the nuclei 

 which combine to form it. In the sexual fusion at the initiation of the 

 ascocarp we find both antheridial and egg nuclei provided with conspic- 

 uous centers. When the pronuclei are lying side by side in the &g^ their 

 centers are also still present. The small size of the nuclei and of the 

 whole sexual apparatus at this stage makes it difficult to trace the stages 

 in the combination of the pronuclei, but the fertilized egg-nucleus shows 

 conspicuously a single center, and it seems probable that here, as in the 

 fusion of the ascus, this center takes its origin in the union of the centers 

 of the fusing nuclei. The permanence of the centers throughout the 

 remaining stages in the life history of the fungus, and especially their 

 definite connection with the chromatin content of the nucleus, makes 

 it highly improbable that either one of them should disappear and be 

 replaced by the other during the process of fertilization. 



In the process of spore formation by free cell division the center is 

 also constantly and conspicuously present, and we are justified in con- 

 cluding that in Phyllactinia the central body is a permanent cell struc- 

 ture maintaining its identity through the whole life history of the plant, 

 involving the varied processes of nuclear division, nuclear fusion, and 

 free cell formation. This, of course, does not necesarily imply the 

 individuality of the center in the sense that it is to be considered an 

 elementary organism or even an organ with complex internal structure, 

 such as we seem bound to conceive is present in the chromosome. The 



