SEXUAL AND, ASEXUAL REPRODUCTION 



249 



FIG. 118. EGGS. A, 

 OF AN ANIMAL; B, 



<fF A PLANT 



between the uniting bodies. It is thought to be a first step 

 toward the true sex which is developed in higher plants. Since 

 the uniting bodies in Mucor are, so far as can be seen, alike, 

 the union is called conjugation. 



Among multicellular animals conjugation 

 is unknown, true sex union being always 

 found instead. 



Fertilization or Sex Union. The eggs 

 of all organisms consist of single cells which 

 have prominent nuclei; Fig. 118. Eggs are 

 usually rounded in shape, although they 

 may vary. In size they range all the way 

 from eggs that are too 

 small to be seen with- 

 out the microscope, up 

 to the size of the ostrich 

 egg. The size of the egg 

 is by no means propor- 

 tional to the size of the animal that pro- 

 duces it. The human egg, for example, is 

 microscopic, and the egg of the hen is gigan- 

 tic in comparison. In large eggs, like those 

 of the hen or the ostrich, the bulk of the 

 egg is made of food material, sometimes 

 called yolk, or deutoplasm (Gr. de^teros = sec- 

 ond + plasma = substance), deposited within 

 the eggshell for the nourishment of the 

 young which is to be developed. The egg 

 has a thin cell wall which is known as the 

 vitelline membrane. 



The eggs of animals are produced in 

 organs called the ovaries; Fig. 119. They 

 are situated in different parts of the body in different animals, 

 and their sole function is to produce eggs, which are then car- 

 ried to the exterior through ducts called the oviducts. As can 



FIG. 119. DIA- 

 GRAMMATIC SEC- 

 TION OF THE 

 OVARY OF AN 

 ANIMAL 



Showing the origin of 

 eggs from the ordinary 

 cells. 00 ova. 



