274 - Multicellular Animals, Especially Man 



TAIL- 



MIDDLE 

 PIECE 



ACROSOME 

 FILAMENT 



VITELLINE 

 MEMBRANE 



CYTOPLASM - 



A 



SURFACE OF 

 JELLY COAT 



FERTILIZATION 

 CONE 



D 



Fig. 15-5. Entry of sperm into an egg (Ho/othuria afro). Note pseudopodial activity at apex of 

 which the acrosome filament and other parts of sperm appear to be engulfed. (Diagram based on 

 tions of Colwin and Colwin, Queens College, New York.) 



cone, by 

 observa- 



engulfs both filament and sperm (Fig. 15-5). 

 And second, the acrosome secretes an en- 

 zyme. This digests part of the surface mem- 

 brane and makes a tiny hole for entrance of 

 the sperm. 



The middle piece of the sperm is also im- 

 portant. It carries a pair of centrioles (p. 

 45), which act as division centers in the 

 cleavage divisions, and it also contains a 

 neatly packaged group of mitochondria that 

 participate in the metabolism of the develop- 

 ing e gg- 



The movements of the sperm are effected, 

 of course, by the tail. This is a typical flagel- 

 lum. The axial filament (Fig. 15-3) displays 

 the same pattern of fibrillar structure (10 

 pairs: 1 central and 9 in a peripheral circle) 

 as is seen in electronmicrographs of cross 

 sections of other flagella and cilia (Fig. 

 11-8). 



Insemination. Most aquatic animals dis- 

 charge their eggs and sperm into the sur- 

 rounding water, and the gametes come to- 

 gether more or less by chance. Such animals 

 produce thousands or even millions of eggs 

 and many times as many sperm. The sperm 

 are usually discharged in the general vicinity 

 of where the eggs are laid, and the tremen- 

 dous number of sperm assures the fertiliza- 

 tion of a high percentage of die eggs. Aqua- 



tic embryos, developing in an unprotected 

 environment, are exposed to many hazards, 

 so that only a small proportion of the off- 

 spring are likelv to reach maturity. 



Most terrestrial animals discharge sperm 

 directly into the reproductive tract of the 

 female, during copulation. In such cases, the 

 number of eggs is sharply reduced, although 

 a multitude of sperm is still the rule. The 

 sperm are extremely small and fragile cells, 

 and even when fertilization occurs within 

 the relatively narrow limits of the female re- 

 productive tract, the distance traveled by a 

 sperm is very great in terms of its own size. 

 Consequentlv, many sperm are required if a 

 few are to succeed in reaching the eggs. 



Activation vs. Fertilization. Contact of a 

 sperm with an egg of the same species stimu- 

 lates the egg to respond almost instantane- 

 ously. Upon such activation, the cytoplasm 

 forms a fertilization cone, from which pseu- 

 dopia reach out along the acrosome filament, 

 engulfing the relatively passive sperm. Some- 

 times, however, the sperm continues lashing 

 its flagellum until the penetration is com- 

 plete. Simultaneously the vitelline mem- 

 brane lifts off from the egg cytoplasm, form- 

 ing the fertilization membrane (Fig. 15-6). 

 In most species the fertilization membrane 

 persists, enveloping and protecting the em- 



