Schwarze: Cleavage in Sporangia 149 



progressive cleavage, by surface furrows and clefts, divides the 

 spore plasm into multinucleated, polygonal blocks of very variable 

 size which round up at once and become the definitive spores. 



Swingle (50) finds cleavage in the sporangia of Rhizopus nigri- 

 cans much like that in Sporodinia, except that the spores are more 

 uniform in size and have thicker walls. In Phycomyces the spore 

 plasm is divided by vacuoles, which become angular and fuse to 

 form irregular clefts. Spore formation is aided by furrows which 

 cut into the spore plasm from the columella cleft. Swingle agrees 

 with Harper as to the method of columella formation. 



Timberlake (52) describes spore formation in the sporangia of 

 the alga, Hydrodictyon, as a progressive cleavage by means of 

 furrows. Percival (41) and Rytz (47), discussing spore forma- 

 tion in Synchytrium endobioticum and Synchytrium succissae, re- 

 spectively, both agree that spore formation is brought about by 

 progressive cleavage. 



In 191 3, Moreau (37) described the spore formation in a num- 

 ber of Zygomycetes. His study may be summarized as follows : 



In Circinella conica spore formation proceeds by means of vacu- 

 oles, which separate fragments of protoplasm having the form of 

 amoebae. The protoplasm contracts around each nucleus, rarely 

 around two nuclei, forming protospores which he compares to 

 those described by Harper for Pilobolus. The nuclei then divide 

 and lead to the formation of multinucleated spores. Moreau states 

 that for Phycomyces nitens and Rhizopus nigricans his observa- 

 tions agree in general with those of Swingle, but on page 32 he 

 refers to the protoplasmic segments as being " amiboide " and con- 

 nected by trabeculae. In Mucor spinescens, Moreau finds that a 

 confluence of vacuoles leads to the formation of elongated proto- 

 plasmic threads. The threads become nodose, each nodosity con- 

 taining one or two nuclei and finally forming a spore. Moreau 

 states that a similar thread stage may be observed in the spore 

 formation in Absidia glauca and Absidia septata. In Syncepha- 

 lastrum cinereum and Syncephalastrum racemosum the spores are 

 said to be formed by a condensation of protoplasm into spherical 

 or elliptical masses, each enclosing one or more spores; generally 

 there is but one nucleus in each spore. Moreau's description of 



