SEXUAL AND ASEXUAL REPRODUCTION 



247 



7 



or less frequency, with sexual reproduction. In the microscopic 

 Animal Hydatina, for example (Fig. 116), found in fresh water, 

 the eggs commonly produced, called summer eggs, develop 

 without fertilization into new females, which rapidly mature 

 and produce more similar eggs that develop in the same way. 

 This may go on for a long time, under proper conditions for 

 hundreds of generations, without any 

 males making their appearance. Even- 

 tually, however, under conditions not 

 yet understood, males make their ap- 

 pearance and the females produce eggs 

 of a different kind, called winter eggs, 

 which are incapable of developing with- 

 out being combined with sperms by the 

 sexual process. Here, then, partheno- 

 genesis seems to be the normal method 

 of reproduction, sexual reproduction 

 alternating with it at unknown and un- 

 certain intervals. The reasons for this 

 alternation, and the conditions that de- 

 termine the one or the other method, 

 are not yet understood. 



FIG. 116. HYDATINA. A 



MICROSCOPIC ORGANISM 

 THAT MULTIPLIES BY 

 PARTHENOGENESIS 



ex, excretory system; 

 gl, gland; 

 m, mouth; 

 st, stomach. 



MULTIPLICATION BY CELL UNION 



Conjugation. In all animals above 

 che unicellular forms, and in most 

 plants, cell union is found as a factor 

 in reproduction. Among a few plants 



of the lower orders the cells which unite are alike. In 

 Mucor, for example, besides the spore formation mentioned 

 on page 97, a union of cells sometimes takes place; Fig. 117. 

 As shown in Figure A, special lateral threads grow out from 

 the ordinary mycelium of the mold, and these come in contact 

 with each other at their tips. After they touch each other 

 single cells are divided off from each, B, which fuse with 



