24 FORMATION OF SPORES OF RHIZOPUS AND PHYCOMYCES. 



the vacuoles after they have entered the sporangium — never, so far as 

 I have observed, appearing in those of the mycelium or sporangio- 

 phore, and rarely in those of that part of the sporangium which lies 

 close to the mouth of the sporangiophore. The younger stages of 

 their formation are shown in PI. IV, tigs. 15 and 16, while in Pis. IV 

 and V, iigs. 18, 19, and 20, they have reached their maximum density. 



As the protoplasm streams up into the sporangium and out toward 

 the periphery, there is at first a gradual transition in density from the 

 center outward, precisely as in MMzoj^us at the same stage. (PI. IV, 

 fig. 15.) A little later, however, as in PI. IV, fig. 17, it is divided into 

 three regions, differing in density. The outer region or layer is very 

 dense and takes the stain strongly. Inside this is a second layer, which 

 is considerablj^ less dense and stains less strongly. Inside this second 

 layer and occup^'ing the central part of the sporangium is a region of 

 very loose and much vacuolated protoplasm which takes the stain 

 scarcely at all. Between the interior region and the second layer the 

 the differentiation becomes very sharp, but, as in Hhizojnis^ there is 

 no wall or membrane of an}" kind between them. Between the second 

 and the outer layers, however, the transition is at first very gradual 

 (PI. IV, fig. IT), but becomes more and more sharp as the sporangium 

 grows older, I have never found in Phyeomyces a stage such as is 

 shown in PI. I, fig. 6, which occurs regularly in Uhlzopus. It is pos- 

 sible that this second layer is homologous with the semitransparent 

 zone that has the same relative position in the sporangium of Rhizoinis. 

 I have not regarded it as such, however, as it is of so much greater 

 relative density and contains no delicate strands representing currents. 

 It is interesting in this connection to compare PI. I, fig. 6, with PI. IV, 

 figs. 17 and 18. 



The nuclei are at first about evenly distriliuted in the outer and second 

 la^^ers, but in the interior there are very few, or for a shoil period in 

 the development of the sporangium none. (PI. IV, figs. 17 and 18.) 



None of the vacuoles in the interior region of very loose protoplasm 

 or in the inner part of the second layer has the stainable content men- 

 tioned above. Practically all of the larger ones in the outer dense 

 layer contain this substance, however, as also do most of those in the 

 outer part of the second layer. (PI. IV, fig. 17.) Between these 

 larger vacuoles are very small ones which contain nothing that takes 

 the stain. (Pis. IV and V, figs. 15-24.) The difference in the destinies 

 of these two kinds of vacuoles will be seen later. 



As may be seen from PI. IV, figs. 15, 17, and 18, and PI. V, fig. 

 19, the vacuoles that contain the stainable substance are very numer- 

 ous, taking up a considerable portion of the space in the sporangium 

 and lying very close together, often two or more being in actual con- 

 tact, their clear zones being separated b}- onlj' the vacuolar mem- 

 branes. (Pis. IV and V, figs. 15, 17, 18, 19, and 20.) In such cases 



