PHYCOMYCES NITENS. 25 



the vacuolar incmhiaiu' is isolated from the remainder of the proto- 

 plasm for a little space, and may readily ])e seen and studied by 

 itself. (PI. V. tio-. 20.) It is very thin and homogeneous, taking the 

 violet stain very sliuhtly, wliich gives it a faint blu-", color. When 

 two vacuoles are thus in contact they are usually flattened against 

 each other, so that the membrane l)etween appears in optical section 

 as a thin, straight line. In such cases the contents are often flattened 

 on that side to conform to the shape of the vacuole. (PI. V, flg. 20.) 

 A considerable number of the nuclei that are in the second layer 

 when it is flrst formed migrate into the denser plasm, and the difler- 

 entiation between the two layers becomes more distinct. Then a 

 layer of vacuoles, practically all having stainable contents, becomes 

 arranged in a dome shape in the denser plasm and running parallel to 

 its inner surface. (PI. IV, fig. 18.) These vacuoles flatten out, 

 become disk-shaped, and fuse edge to edge to form a dome-shaped 

 cleft in the denser plasm, as in Rhizopm and Plloholus. (PI. V, fig. 

 19.) It is interesting to note that as the vacuoles flatten, the content 

 flattens also, so that its surface remains always more or less parallel 

 to the vacuolar membrane. (PI. V, fig. 19.) 



So far as I have been able to observe, there is never a surface fur- 

 row that cuts inward to meet the lowest of the layer of vacuoles, as is 

 the case in FUoholux and Rhizopm. In this respect Phycomyces 

 appears more like Sjxyrodlnia. The layer of vacuoles begins so very 

 near the surface of the protoplasm (PI. V, fig. 19) that if there is 

 such a surface furrow it nnist l)e very shallow indeed. I have never 

 found any evidence of its existence. 



When the vacuoles of this layer have entirely fused, edge to edge, 

 the separation of the columella is complete. There is at first no wall — 

 simply a cleft bounded l>y plasma-membranes. The contents of all 

 the vacuoles that make this cleft have now fused, forming a layer of 

 slightly uneven thickness separating the outer surface of the columella 

 plasm from the inner surface of the spore-plasm. All the very loose 

 interior protoplasm, the second layer, and a small part of the denser 

 plasm are included within the columella, while the greater part of the 

 denser plasm goes to form the spores. 



As soon as the difl'erentiation of the columella is complete, or in 

 exceptional cases a little before, the formation of the spores begins. 

 Here we get a most striking diflerence between Phycomyces and 

 Rhizopm. The large round vacuoles in the spore plasm begin to lose 

 their rounded form and become angular. (PI. V, figs. 21 and 22.) 

 These angles become sharper and sharper, and appear to cut through 

 the cytoplasm between the nuclei, and when they encounter each other 

 fuse to form irregular clefts. The cytoplasm in advance of these vacu- 

 olar furrows shows no visible difl'erentiation, but remains of an even 

 density throughout the entire spore-plasm during the whole process of 



