The Development of "the Vertebrate Embryo 53 



ACTIVATION. As mentioned previously, the egg as it develops in the 

 female stops at one or another stage of meiosis and lies dormant until 

 activated by contact with the sperm. A number of characteristic changes 

 then occur: 



1. Completion of meiosis. The nuclear membrane breaks down and 

 the egg nucleus completes the meiotic process ( in eggs where it is incom- 

 plete ) . 



2. Fertilization cone. In some eggs a conical projection pushes up 

 through the surface jelly from the egg membrane. It "captures" the sperm 

 at the jelly surface and by contraction draws it down toward the egg. 

 Meanwhile, the sperm dissolves a portion of the membrane to permit 

 ingress. 



3. Block to polyspermy. Penetration by the first sperm makes most 

 eggs almost instantaneously impervious to the entrance of any others. 

 The mechanism of this reaction is still unclear. In a few species, however 

 (some reptiles and insects), the entrance of more than one sperm is a 

 normal event. One of the sperm nuclei fuses with the egg nucleus and 

 the others degenerate and disappear. 



4. Rearrangement in egg contents and physiology. Upon fertiliza- 

 tion the egg changes its shape, generally veering toward the spherical. 

 The permeability of the egg membrane to the ingress of salts and other 

 substances rises markedly. Cytoplasmic materials migrate to new areas. 

 The axes of the future embryo ( dorsal- ventral, anterior-posterior) are 

 fixed. 



Many eggs can be activated in the absence of sperm. Among treat- 

 ments found successful are: X-ray and ultraviolet irradiation, introduction 

 of foreign protein by a needle, heat shock, addition of certain salts and 

 acids. The activated egg proceeds through the changes noted above and 

 goes on to cleave and construct an embryo. The eggs of certain organisms 

 (sea urchins, starfish, frogs, bony fishes, rabbits) can develop perfectly 

 normally without fertilization by sperm and give rise to viable adults. The 

 process, called parthenogenesis, is carried out routinely in nature by colo- 

 nial insects including bees. Although only the egg nucleus is present, the 

 cells of a parthenogenetic embryo need not remain haploid. In some cases 

 the activated haploid egg duplicates its chromosomes without cell divi- 

 sion, thereby producing a diploid, homozygous individual. 



FUSION OF NUCLEI. The sperm head and midpiece enter the egg, 

 membranes disappear, and the sperm chromosomes arrange themselves on 

 a spindle constructed by material in the midpiece (Fig. 26). Sperm and 

 egg nuclei migrate toward each other and the egg chromosomes arrange 

 themselves on the spindle. The pattern now resembles that seen in meta- 



