426 MOSSES AND FERNS CHAP. 



spermatozoids collect in equal numbers about those which are 

 still closed, and even about spores that have not germinated 

 at all. Marsilia did not prove a good subject for studying the 

 behaviour of the spermatozoid within the egg, owing to the 

 difficulty of differentiating the spermatozoid after its entrance. 

 Pilularia is better in this respect, and shows that the changes 

 are the same as those described in Marattia and Osmunda. 



Coincident with the first divisions in the embryo, each of 

 the lateral cells of the prothallium (venter) divides by a peri- 

 clinal wall, but the basal layer of cells remains but one cell thick. 

 The prothallium grows with the embryo for some time, and in 

 its later stages develops abundant chlorophyll, and its basal 

 superficial cells grow out into colourless rhizoids. In case the 

 archegonium is not fertilised, the prothallium grows for a long 

 time, and reaches considerable size, but never develops any 

 secondary archegonia. In Pilularia, both prothallium and em- 

 bryo may develop chlorophyll in perfect' darkness (Arcangeli 

 (i),p. 336). 



The Embryo (Hanstein (2); Campbell (3, 15)) 



The two genera correspond very closely in the development 

 of the embryo, which shows the greatest resemblance to the 

 Polypodiacese. In Marsilia the development of the embryo 

 proceeds very rapidly. The first division of the egg is com- 

 pleted within about an hour after the spermatozoid enters, and 

 in Pilularia after about three hours, as nearly as could be made 

 out. In both the basal wall is vertical and divides the some- 

 what flattened egg exactly as in Onoclea. The quadrant walls 

 next follow, and then the octant wall, as usual. Of the latter 

 the one in the root quadrant diverges very strongly from the 

 median line (Fig. 250, C), and that in the foot quadrant is 

 much like it. In the others it is nearly or quite median, anc' it 

 is impossible to say which of the leaf and stem octants is to 

 form the apical cell of those organs. The relative position of 

 the young organs is exactly the same, both with reference to 

 each other and to the archegonium, as in the Polypodiacese. 



The Cotyledon 



The cotyledon grows for a time from the regular divisions 

 of one or both of the primary octant cells, but this does not 



