juiyis, I9I5 Rheosporangium Aphanidermatus 281 



the side branches, which now present the appearance of swollen pro- 

 tuberances (PI. XLIV, fig. 7). 



The time at which the rupture of a given presporangium may be 

 expected to occur can be predicted with reasonable accuracy some little 

 time in advance by the appearance of the vacuoles, especially by one 

 which develops at the tip so as to produce a relatively large hyaline 

 area at the point where the break is to occur. This fact makes it easy 

 to observe, with the oil- or water-immersion lens, the expulsion of the 

 contents and its subsequent cleavage from the beginning, because in 

 hanging-drop cultures the field within which it is to appear may be 

 brought into focus before the phenomena begin to develop. 



When the rupture occurs, there is at first a rush of protoplasm from 

 the presporangium, soon to be modified to a steady flow of diminishing 

 velocity. The entire protoplasmic contents flow out, leaving only the 

 empty wall. The new body thus formed may be seen to be inclosed in a 

 thin, membranous, almost invisible and very plastic wall, which is so 

 flexible that the discharging mass takes on a spherical form as it is re- 

 lieved from the pressure of the presporangium wall. During egress, 

 therefore, the delivered portion presents the appearance of an enlarging 

 sphere of protoplasm, while during the later part of the process the 

 inclosed portion of the membranous wall may be seen advancing along 

 the presporangium cavity, drawing out with it the last portions of the 

 contents. This body, which is a zoosporangium, remains at the mouth 

 of the presporangium wall during cleavage, although it does not appear 

 to be attached to it by any visible means. Promptly following the 

 egress of the sporangium, its cytoplasm cleaves into zoospores, which 

 are liberated by the rupture of the sporangium wall. 



The various steps in the process have been followed in the living 

 material in hundreds of cases, as well as in the sections. In a typical 

 instance observed in a hanging-drop culture under a water-immersion 

 lens the liberation from the presporangium was completed at 11 a. m. 

 Cleavage lines first appeared as indistinct grooves at 1 1 .07. Short cilia 

 were observed waving at the periphery at 11.09, and s-t 11. 10 a rocking 

 motion of the entire mass began. This continued uninterruptedly dur- 

 ing the remaining time. The spores began to assume a definite outline 

 at 1 1. 1 3 and exhibited individual motion at 11. 17. A large vacuole in 

 each spore had become distinctly visible at 11. 18. The spores began to 

 change their relative positions quite freely at 11.25. They had assumed 

 the normal adult shape at 11.26 and were swimming about with great 

 activity within the sporangium. They escaped at 1 1.29 through a punc- 

 ture produced in the membrane by the force of their impact upon it. 

 In this instance 24 spores were counted. The number was commonly 

 somewhat larger, approximating 50, although instances of as few as 4 

 were seen, and sporangia containing a number estimated to be 150 or 



