. Principles of modern biology. Biology. 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 o
. Principles of modern biology. Biology. 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 egg- 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 extr
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