44 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



unable to determine. In the case of reduction divisions it would seem 

 probable that the fibers merely separate in two groups. There is no 

 trace of a direct connection between the centers as they separate. There 

 is no evidence of the existence of a differentiated central spindle at this 

 stage. As I have suggested (38) for Erysiphe, it is possible that, for 

 a time at least, the presence of the nuclear membrane makes a central 

 spindle unnecessary. As to the permanence of the nuclear membrane 

 and the absence of a central spindle in the early stages of the separation 

 of the centers, the conditions in the ascus are similar to those in the 

 spindle formation in the cleavage of the egg of the trout, whitefish, 

 and many invertebrate animals in which the centers migrate to opposite 

 poles of the nuclei before the nuclear membranes disappear. It has, 

 however, been rather generally assumed by students of karyokinesis in 

 these eggs that the connection of the centers to the chromosomes is 

 established by the growth of spindle fibers into the nuclear cavity after 

 the centers have reached their position at the poles. It is interesting 

 to note in this connection, however, that Janssens (49) claims, on the 

 basis of an investigation of the spindle formation in Triton, that even 

 here no central spindle is formed between the separating centers as 

 described by Hermann (43) for the salamander. 



The centers continue to separate until they have passed through 

 an entire half-circle and come to lie opposite each other, forming the 

 poles of the spindle. The fibers attached to the chromosomes have 

 shortened at the same time and have drawn the chromosomes up into 

 the middle region between the poles (figs. 53, 54). The nuclear mem- 

 brane may be still intact in some cases (fig. 67^) at this stage in Phyl- 

 lactinia, as it regularly is in Erysiphe. In other cases it seems to have 

 entirely disappeared (figs. 53, 54, 670). A remnant of the nucleole 

 may still be present at the equatorial plate stage (fig. 53). 



The chromosomes are oblong or oval bodies in the equatorial plate 

 stage and stand frequently in a radial position on the spindle that is, 

 attached to the spindle fibers at one end and with their long axes at 

 right angles to the long axes of the spindle. They are small as com- 

 pared with the size of the spindle and generally lie entirely free from 

 each other, so that they can be very readily counted (fig. 53). The 

 number is quite regular and corresponds with the number of strands 

 attached to the central body in the spirem stage. These figures agree 

 with those I have already published for other Ascomycetes in showing 

 the incorrectness of Maire's (61) and Dangeard's (21) claim that the 

 Ascomycetes have regularly four chromosomes. The spindle fibers 



