186 



BIOLOGY: GENERAL AND MEDICAL 



free side, pores soon open here and there upon the sur- 

 face, the internal cells throw out cilia into the para- 

 gastric cavity, and a sponge is formed. 



The reproduction of the ferns is of considerable interest 

 because these plants again show a peculiar form of con- 

 jugation effected by male and female cells of common 



Fio. 79. The fern prothallium and archegonium. A, Stages in the germi- 

 nation of the spore; B, young prothallium, showing first appearance of wedged- 

 nhaped, apical cell x; C, tip of prothallium beginning to take on the heart-shaped 

 form; x, apical cell; D, mature prothallium, showing group of archegonia on the 

 cushion just back of the notch, and antheridia further back; rh, rhizoids; E, 

 an open archegonium with egg ready for fertilization and two sperms near the 

 entrance of the neck. (A, B, C, E, after Campbell; D, after Schenck.) (From 

 Bergen and Davis' "Principles of Botany." Ginn & Go., publishers.) 



parentage. Thus the fern produces a large number of 

 spores which, however, do not grow into ferns, but under 

 favorable conditions develop into a peculiar thin, fleshy, 

 heart-shaped mass, called the prothallium. This always 

 remains small, not exceeding one or two centimeters in 

 diameter. The greater part of it is purely vegetative, 



