442 SECOND PART. MYCETOZOA. 



and being of the average size of from 5 to 10 /*. These spore-heaps resemble small 

 sporangia of a Myxomycete, but have no distinct wall ; the spores are only held 

 together and surrounded by a slight structureless enveloping substance. 



In the Ghittulinae, which have been carefully examined by Cienkowski and Fayod, 

 the development, apart from the possible resting-states which will be described below, 

 is limited to the above phenomena. The ripe spore-heaps are formed only of spores. 

 They lie on the substratum as round or elongated bodies of the size of a pin's head 

 and with the appearance of small white, yellow or red drops. 



In the other genera of the group Dictyostelium and Acrasis a division of labour 

 appears among the heaped up swarm-cells. When the body which is formed by them 

 begins to rise above the substratum, those which are in its central line form firm 

 outer membranes at the expense of their protoplasm, and are gradually changed into 

 chambers formed of cellulose and filled with a transparent substance. They remain 

 in close union with one another without interstices, and being built up on or against 

 one another in a single row or in several rows in large Dictyostelia, they form a stalk 

 which rests on the substratum and traverses the middle of the body somewhat in the 

 manner of the stalk in the sporangium of Stemonitis (Fig. 186 a). This stalk grows 

 for a time acropetally by addition of new elements. The aggregate of swarm-cells 

 surrounding the stalk lengthens in proportion as the stalk grows and becomes corre- 

 spondingly narrower, and ultimately separates from the substratum as the stalk ceases 

 to grow ; it then creeps up the stalk leaving it bare and proceeds to form its spores at 

 the apex. This process also is to some extent illustrated by Fig. 186 which represents 

 Stemonitis. In Dictyostelium the spore-masses borne on the stalk resemble in the main 

 those of the Guttulineae. According to Van Tieghem the spores are arranged in 

 Acrasis in rows one above another like beads in a rosary. For further details the 

 reader is referred to Van Tieghem's and Brefeld's publications. 



In Guttulina protea, Fayod found that the swarm-spores grown in a fluid and 

 remaining isolated may undergo the change into spores of the form and structure 

 which they have normally in the state of aggregation. When the conditions are 

 unfavourable for the development these swarm-cells form a thick outer membrane in 

 a complicated manner, and pass into a state which corresponds to the encystment of 

 the swarm-cells of the Myxomycetes (page 427) and may therefore be termed a 

 transitory resting-state. Van Tieghem observed a different mode of encysting in 

 Acrasis and Dictyostelium when the conditions were unfavourable ; a swarm-cell put 

 out a number of processes or arms one after another, which separate from the 

 parent-cell, round themselves off and become invested with a membrane. 



In Brefeld's first work on Dictyostelium the development was to some extent 

 incorrectly described, the dense aggregations of swarm-cells which develope into 

 the stalked spore-masses being supposed to be plasmodia, that is, products of the 

 coalescence of swarm-cells, and the rest of the phenomena being interpreted in ac- 

 dance with that supposition. Van Tieghem's correction of this mistake in 1880 was 

 cautious but at the same time clear and complete, and Brefeld has recently (1884) 

 published a full confirmation of this correction. 



AFFINITIES OF THE MYCETOZOA. 



SECTION CXXIV. In investigating the affinities and homologies of the Mycetozoa 

 which have now been described, we must distinguish between the Myxomycetes which 



