382 



MUSCINEJE. 



mella. The intercellular space is traversed by rows of cells which stretch across from 

 the wall of the theca to the inner mass of tissue ; they resemble most nearly proto- 

 nemal filaments, or those of Algae, but have been formed by simple differentiation of the 

 tissue of the theca. They contain chlorophyll-granules like the inner cell-layers of 

 the wall. The outer layer of the wall of the theca is developed into a very character- 

 istic epidermis strongly cuticularised externally. The third or fourth layer of cells of the 

 inner mass of tissue, which is therefore separated from the annular air-cavity by two or 

 three layers of cells (forming the spore-sac), produces the mother-cells of the spores. 

 The cells of this layer are first of all distinguished by being densely filled with proto- 



FlG. 268. — Punarta hy^^rometrica ; 

 transverse section through the spore- 

 sac ; A, sti the primary mother-cells ; 

 B, sm the spore-mother-cells not yet 

 isolated ; a outer side, i inner side of 

 the spore-sac_{x 550). 



Fig. 269.— Development of the spores of Fimaria hygrometrica observed in 

 very dilute glycerine ; A mother-cells, at a still united, at b and c the separation has 

 commenced; ^isolated mother-cells clothed with cell-walls; at y expelling the 

 protoplasmic contents ; C mother-cells with indication of the commencement of 

 the bipartition of the contents ; D the contents have divided into four masses of 

 protoplasm, still surrounded by the prnnary cell-wall, but they themselves are 

 naked ; E the spores enveloped by cell-walls ; F ripe spores (X 550). 



Fig. 270.— Various states of division of the mother-cells of the spores oiFtmaria 

 hygrometrica, observed in water, the progress of development indicated by the 

 letters a—i. 



plasm, in which lies a large central nucleus, and are attached without interstices to the 

 surrounding tissue in a parenchymatous manner. From their division proceed the spore- 

 mother-cells, which become isolated by the deliquescence of the cell-walls, and then float 

 in the fluid contained in the spore-sac, till they form the spores by repeated division. 

 The Spore-sac is the term given to those layers of cells by which the large air-cavity is 

 separated from the spore-mother-cells. It seems convenient to consider the layers 

 which bound the spore-cavity on the axial side (Fig. 268, i) also as a part of the 

 spore-sac; its cells contain on both sides starch-forming chlorophyll-granules. The 

 inner large-celled tissue, which contains but little chlorophyll, and is thus surrounded 



