250 



BIOLOGY 



be seen from Figure 119, the egg is really a single cell, like the 

 other cells of the body in structure, though larger in size. As 

 the egg passes along the oviduct it is not infrequently sur- 

 rounded with a mass of yolk and a shell; neither the yolk nor 

 the shell is an essential part of the egg, the yolk being a food 

 material for the nourishment of the embryo, and the shell be- 

 ing a covering to protect the egg after it has left the body. 



Plants also produce eggs similar in structure to those of 

 animals (Fig. 118 B), though the organs that produce them 

 are not called ovaries.* 



Sperms. Sperms are extremely minute cells which must 

 unite with the eggs in order that the latter may be capable of 



further development. Sperms are by 

 no means uniform in shape. As a 

 rule, each consists of a minute head 

 and a motile tail, whose lashing move- 

 ments propel the sperm through 

 liquids until the sperm is brought in 

 contact with the egg. Figure 120 

 shows the sperms of a number of ani- 

 mals and plants. There is great vari- 

 ety among them, and, while some of 

 them are provided with tails, others 

 are not, and, although usually motile, 

 the sperms of some animals are sta- 

 tionary. The sperms of animals are 

 produced in special glands called sper- 

 maries or testes. In the frog and 

 earthworm the position of these sperm 

 glands is shown in Figure 80. The sperms are passed from 

 the spermary into ducts, commonly known as the vasa defer- 

 entia, which carry them to the exterior. These ducts may be 



* It will be noticed that the ovary of an animal is quite different from 

 the ovary of a flower, since the latter does not produce eggs nor have 

 oviducts; see page 273. 



A B 



FIG. 120. VARIOUS FORMS 



OF SPERMS 



A, B, C, D, and E, sperms of 

 animals; F, of a fern; G, of a liver- 



wort. 



(Various authors.) 



