294 THE PHJiNOMENON OF 



SO that the latter become diffused, or form an irregular 

 accumulation between the two mother-individuals, which 

 of course frustrates the formation of the spores. In 

 normal development the whole mass of green contents 

 enters in a short time into the two connecting canals, 

 which swell up from the middle, where they are in con- 

 tact, out, in opposite directions, into obovate or almost 

 globular sacs, which wedge themselves in closely between 

 the four legs or horns of the conjugated individuals. In 

 each of these sacs the contents become balled into a 

 globe, which becomes clothed with a cell-membrane, at 

 first delicate and smooth, afterwards thick and granulated. 

 In this way, therefore, two spores are formed, completely 

 separate in their origin, which so closely fill up the deli- 

 cate sac-like canals of union in which they are formed, 

 that as they lie between the mother-individuals, they 

 appear like free globules, but are persistently connected 

 with the mother-cells by the empty and colourless lateral 

 parts of the two closely approximated uniting-canals, 

 which extend into the valve-like openings of two empty 

 shells. These connecting pieces, formed from the inner 

 membrane, are readily distinguished, by their want of 

 colour, from the outer membrane or shell, which appears 

 straw-coloured after the contents are emptied out. A 

 not unfrequent abnormity affords a further proof of the 

 origin of the two spores through conjugation of opposite, 

 completely distinct pairs of cells ; this is the occurrence 

 of only one spore between two mother-individuals. In 

 this case only two corresponding halves (or horns) of the 

 mother-individuals become emptied, while the two others 

 remain densely filled with green contents, which, in spite 

 of the dehiscence of the outer cell-membrane, cannot flow 

 out. 



b. Conjugation in crossed {decussate) position. — This 

 occurs in the entire group of forms allied to Euastrum, 

 and also in many genera formerly united with Des- 

 midium. 



a. The halves of the dehiscent cells separate by an 



