40 INTRODUCTION. 



and thus enclose all the protoplasm in the sporangium, is an interesting ques- 

 tion. The necessity is evident that the cleavage should proceed through a 

 tolerably dense plasma, and this is, perhaps, due to the need of two proto- 

 plasmic surfaces in contact in order to form a cell-wall. 



The fact that the columella is not deposited on the surface of the 

 central vacuole seems to indicate that the limiting layer of a vacuole 

 is not quite a plasma membrane, although it may partake partly of 

 the real nature of one. Although there is much to show that the wall 

 of a vacuole, such as we are dealing with here, and a plasma mem- 

 brane are closely related, yet the author is not quite ready to admit 

 that they are the same. Why two plasma membranes should be in 

 contact in order to form a cell- wall, as suggested by Harper, is not 

 quite clear to the author, since in many cases a single' plasma mem- 

 brane will secrete a cell- wall. 



In the cleavage of the spore-plasma, which begins soon after the 

 columella is complete, vacuoles also take an important part. The 

 cytoplasm becomes somewhat vacuolar, and the numerous nuclei are 

 rather evenly distributed throughout its mass. Cleavage furrows 

 appear now near the base of the sporangium, cutting the surface into 

 irregular polygonal areas (Fig. 15, B). At the same time vacuoles 

 in the interior become angular, appearing three-cornered in section, 

 and their edges cut through the cytoplasm to meet similar cleavage 

 furrows from adjacent vacuoles (Fig. 15, B). In the meantime the 

 surface furrows which have been growing deeper meet and become 

 continuous with the edges of the vacuoles. By pressure of the adja- 

 cent plasma-masses, the surfaces of the vacuoles which were formerly 

 convex become concave, and the vacuoles appear as intercellular 

 spaces between the cleavage-segments. In this manner the spore- 

 plasma is marked out into irregular blocks, apparently without refer- 

 ence to the size or number of nuclei they contain. A continuation of- 

 the process cuts the spore-plasma into oblong rounded sausage-shaped 

 masses containing generally two to four nuclei in a row (Fig. 15, C). 

 These oblong masses now divide transversely to form rounded bodies 

 with one or few nuclei (Fig. 15, D). This completes the primary 

 cleavage by which the spore-plasma has been cut up into smaller 

 units with one or few nuclei. These units are not the spores. They 

 undergo a period of growth and nuclear division before the final 

 cleavage divisions take place by which the mature spores are pro- 

 duced. The last divisions are, however, similar to the first, presenting 

 the simpler process of cleavage or fission. 



In the sporangium of Pilobolus^ we have a cleavage which is of 

 the same type as in Synchitrium, with the exception of the promi- 



