62 



GENERAL CONCEPTS 



Corona. Rad.ia.ta. 



Nucleolus 

 Nucleus 



Zona Pellucida. 



Middle /^He^d 

 piece- \ I 



Figure 3.16. Human egg and sperm magnified 400 X. Inset, side and top views of 

 a sperm, magnified about 200 X. The egg is surrounded by other cells which form the 

 corona radiata. 



1 8. Body Plan and Symmetry 



To refer to the regions of an animal body, zoologists use the term 

 anterior for the head end and posterior for the tail end; the back side 

 is the dorsal side and the belly side is the ventral side. The midline of 

 the body is medial and the sides are lateral. The part of a structure 

 nearer the point of reference is proximal, the part farther away is distal. 



A body is symmetrical if it can be cut into two equivalent halves. 

 A few kinds of protozoa can be cut into two equal halves by any 

 plane through the center; they are said to be spherically symmetrical. 

 Coelenterates and echinoderms are radially symmetrical; they can be 

 cut into two equal halves by any plane which includes the axis run- 

 ning from top to bottom through the center. In such animals a top 

 and bottom side can be distinguished. Most other animals are bilaterally 

 symmetrical, and can be cut into two equivalent halves only by a plane 

 passing from anterior to posterior and from the dorsal to ventral sides 

 in the midline. In such a bilaterally symmetrical animal, three types of 

 planes or cuts can be made to get different views: sagittal, frontal and 

 transverse (Fig. 3.17). A sagittal section is one made by cutting in the 

 median vertical plane; thus it includes the anterior-posterior axis and 

 the dorso-ventral axis but is at right angles to the right-left axis. A 

 frontal section is at right angles to a sagittal section and includes the 

 anterior-posterior axis and the right-left axis, but is perpendicvdar to 

 the dorso-ventral axis. Transverse sections are cut at right angles to the 

 anterior-posterior axis and include a dorso-ventral and a right-left axis. 



