RHIZOPUS NIGRICANS. 17 



as a bundle of ytnuicLs, which in optical section spread fan-like as they 

 enter the sporangium and extend toward the periphery. Many of 

 these streams, particularly at the sides, extend nearly to the sporan- 

 giunj wall, as seen in PI. I, tig. 5. Harper (lSiM») has described each 

 individual current in Plloholm as having ''marked a path for itself 

 through the protoplasmic structure. It is marked by continuous deli- 

 cate films, (piite distinct from the spongy structure of the adjacent 

 plasma." These surrounding tilms, as the writer has seen them in 

 Bhisopm, are of a more hyaline and homogeneous appearance than 

 either the currents or the surrounding cytoplasm. 



The nuclei in these currents are much elongati^d in the direction in 

 which the currents run and I have not been able to differentiate the 

 parts, tJie nucleolus and chromatin l)oth staining red. 



As the streaming contiiuies the protoplasm at the periphery becomes 

 denser, and there appear clearly differentiated layers within the spo- 

 rangium. The beginning of this differentiation is not simultaneous 

 throughout the protoplasm. It appears at certain points approxi- 

 mately equidistant from the periphery, between which and the periphery 

 the thickening of the protoplasm forms a dense zone lining the sporan- 

 gium except at the base, where its inner boundary line gradually extends 

 to the periphery. In the stage shown in PI. I, tig. 0, this boundary line 

 is not perfect, but somewhat l)roken, admittijig thin streams of loose 

 protoplasm from the interior of the sporangium. Inside this zone 

 and of about one-third its thickness is a semitransparent layer consist- 

 ing of loose protoplasm like that which tills the interior of the sporan- 

 gium, but clearer and less granular, and taking the orange stain less 

 strongly than the latter. In structure it resembles the thin tilms about 

 the streams previously mentioned. The cytoplasm inside this semi- 

 transparent zone and occupying the central and lower part of the 

 sporangium is of a loose, spongy, much-vacuolated structure, contain- 

 ing scattering nuclei and a considerable number of proteid bodies. 

 There are no marked protoplasmic strands indicating currents in the 

 center of the sporangium, though the writer has often found them in 

 later stages; but radiating from the central part of the sporangium 

 and passing from it through the clear zone to the denser plasm are 

 many very slender strands marking the paths of currents. These 

 currents bear nuclei and seem to represent a very late stage of the 

 migration of cytoplasm and nuclei toward the periphery. Some of 

 these streams enter the openings in the denser plasm, while others run 

 against its inner surface. This streaming goes on until the inner 

 boundary of the denser plasm is at all points sharply detined. This 

 boundary does not consist of a membrane or of any differentiated layer. 

 The denser plasm at this time contains only a very few vacuoles of 

 any considerable size, but under very high magnification it can be seen 

 20844— No. 37—03 2 



