48 The Development of the Vertebrate Embryo 



can trigger or curtail the next and therefore provide chronological order 

 to what would otherwise be chaos. 



In the rest of this chapter we shall consider these processes, as well 

 as the earlier phases of embryogenesis, i.e., the formation and union of 

 the gametes, in some detail. 



The Early Stages of Embryogenesis 



THE FORMATION OF GAMETES: THE SPERM 



The first step in the recipe for roast chicken is: find your chicken. 

 Similarly, nature's instructions to a sperm are: first, find your egg. To 

 accomplish this, the mature sperm is a stripped-down model equipped 

 with organs of locomotion and with a high rate of metabolism so that it 

 can generate energy to move rapidly over relatively great distances. Once 

 in contact with the egg, it must dehver a nucleus containing a haploid 

 set of chromosomes, one-half the complement of the future embryo. Fig- 

 ure 23 is a drawing of the sperm of several animals and includes a recon- 

 struction of what we see in the electron microscope when we look at the 

 midpiece of ram sperm. This should give you some idea of the sperm's 

 structural complexity. Various sperm may look very different, but they all 

 contain these basic parts: 



Fig. 23. Spermatozoa. 



Fine structure of mid- 

 piece as revealed by 

 electron microscope 

 after Woddington) 



Human Fowl Salamander Re 



Crustacea 



