MICHIGAN ACADEMY OF SCIENCE. 67 



and sperms). The union of the gametes gives rise to a zygote with 

 a diploid nucleus, the subsequent nuclei all being diploid until 

 meiosis occurs at gamete production again. The sperm mother 

 cell possesses a diploid nucleus. By meiotic division it produces 

 lour haploid nuclei, one for each of the four sperms. Within the 

 ovary or after it leaves the ovary the egg mother cell also has a 

 diploid nucleus. The so-called ''maturation" divisions that it 

 makes are in reality meiotic and the result is four eggs, of which 

 only one is functional, the other three being the polar bodies. 

 (It must be noted that frequently the first polar body does not 

 divide again, in which case only two polar bodies are apparent.) 



Illustrating the sexual cycle schematically as a circle (Fig. 3) 

 we have CU representing the union of the gametes and NU the 

 immediately following nuclear union. The portion of the circle 

 around to the letters RD (reduction division) represents the long 

 stage of somatic development in which nuclear and cell divisions 

 occur until the reduction divisions take place at the time of form- 

 ing the next generation of gametes. 



Turning now to the Vegetable Kingdom we find that there is 

 no such general uniformity of the sexual cycle as we find in animals. 

 The gaps between the different events of the cycle may occur be- 

 tween any two, or there may be two gaps. It will be understood 

 that these "gaps" represent series of nuclear and cell generations 

 between one point of the cycle and the next. 



To find a sexual cycle of the type that is prevalent in animals 

 we must turn to Fucus (Fig. 3). Here the gametes (produced at 



Figure 3. Sexual cycle in Metazoa and in Fucus. RD= Reduction division, CU=Cell union, 



NU=NucleaT union, G=Point of gamete formation. 

 Figure 4. Probable sexual cycle of Ulothrix, Oedogonium, Desmidiaceae, Spirogyra, etc. 



