504 



The Determination of Sex 



phyte stage does not appear, for the spermatids usually develop 

 directly into the spermatozoa and the ootids become the eggs. 

 Fertilization then reconstitutes the zygote in each organism. 

 These relationships are shown diagrammatically in Fig. 144. 



Chlorophyceae 



zygote 



resting spore 



i. 



(meiosis) 



I 



spores 



Embryophyta 



zygote 



sporophyte • 



A 



(meiosis) 



\ 



microspores megaspores- 



Animals 



• zygote 



Y 

 • soma 



A, 



(meiosisj 



/, \ 



-spermatids ootids 



gametophyte — microgametophyte megagametophyte 



sperm eggs 



zygote- 



sperm 



eggs- 



zygote 



• sperm 



'I 

 eggs 



■zygote 



Fig. 144. Diagrams of life cycles of the Chlorophyceae, Embryophyta, 

 and animals. In the Embryophyta the zygote develops into a multicellu- 

 lar structure, the sporophyte; in animals, this multicellular structure is the 

 soma or body; in the green algae (Chlorophyceae) such a multicellular 

 structure does not develop but the zygote becomes merely a resting spore. 

 The products of meiosis in the Chlorophyceae and Embryophyta develop 

 into a multicellular structure, the gametophyte, part of which subsequently 

 produces the gametes by mitosis. In animals, the immediate products of 

 meiosis (spermatids and ootids) do not divide into a multicellular struc- 

 ture but merely differentiate into the sperm and eggs. 



Sex Chromosomes 



Plants which might be thought most likely to resemble ani- 

 mals in their sexual behavior are the dioecious type since in these 

 plants each diploid individual is either a male or a female and 



