DEVELOPMENT OF SPOROGONIUM 333 



takes on the character of an ovum (B 2 , ov\ the others, called 

 canal-cells (en. c) are converted into mucilage, which by its 

 expansion forces open the mouth of the flask and thus makes 

 a clear passage from the exterior to the ovum (B S ). 



Through the passage thus formed a sperm makes it way 

 and conjugates with the ovum, producing as usual an 

 oosperm or unicellular embryo. 



The development of the embryo is at first remarkably 

 like what we have found to take place in Hydroids (p. 245). 

 The oosperm divides into two cells by a wall at right angles 

 to the long axis of the ovary : each of these cells divides 

 again repeatedly, and there is produced a solid multicellular 

 embryo or polyplast (c 1 spgnm). 



Very early, however, the moss-polyplast exhibits a striking 

 difference from the animal polyplast or morula : one of its 

 cells that nearest the neck of the ovary takes on the 

 character of an apical cell, and begins to form fresh seg- 

 ments like the apical cell of the stem. Thus the plant 

 embryo differs almost from the first from the animal embryo. 

 In the animal there is no apical cell : all the cells of the 

 polyplast divide and take their share in the formation of the 

 permanent tissues. In the plant one cell is at a very early 

 period differentiated into an apical cell, and from it all cells 

 thereafter produced are, directly or indirectly, derived. 



The embryo continues to grow, forming a long rod-like 

 body (c 2 , spgnm) the base of which becomes sunk in the 

 tissue of the moss-stem, while its distal end projects vertically 

 upwards, covered by the distended venter (v) of the ovary. 

 Gradually it elongates more and more and its distal end 

 dilates : the embryo has now become a sporogonium^ con- 

 sisting of a slender stalk (c 4 , st) bearing a vase-like capsule 

 or urn (?/) at its distal end. In the meantime the elonga- 

 tion of the stalk has caused the rupture of the enveloping 



