FERTILIZATION 325 



By experiment it can be shown that the sperms of mosses are 

 attracted by solutions of cane sugar and fern sperms are at- 

 tracted by solutions of maHc acid. These or similar sub- 

 stances are probably excreted by the egg and serve as a direct- 

 ing stimulus to the sperm. 



701. Many eggs are enclosed in a gelatinous envelope, which 

 is readily penetrated by the sperm. In eggs which have a firmer 

 covering, or ^^ shell," there are one or more pores (micropyles) 

 through which the sperms enter. In most cases only one sperm 

 cell enters the egg under normal conditions. This control is 

 effected by the response of the egg to the first sperm. For no 

 sooner has a sperm entered the egg than the latter develops a 

 membrane which excludes all other sperms. 



702. Cleavage. — After fertilization the eggs of higher plants 

 and animals immediately begin segmenting (early cell division). 

 Among the lower forms, on the contrary, a long ''resting stage" 

 often ensues after the zygote (conjugated gametes) or fertilized 

 egg has formed a heavy membrane, 



703. In some instances the chromosomes of the two gametes 

 can be separately followed through the first cell division. In 

 which case it is seen that the chromosomes of the first segmen- 

 tation spindle come in equal numbers from the two gametes 

 and that therefrom the daughter cells receive an equal number 

 of chromosomes from each parent. This is an important point 

 which will be discussed more fully at another place. The events 

 which follow the first cell division vary greatly with the organ- 



first division of the micronucleus; D, second division; E and F, three daughter 

 nuclei disintegrate, the fourth divides into a migrating male element and a pas- 

 sive female element; F and G, the male element migrates into the other cell and 

 fuses with the opposite female element; //, the process completed; /, the cells 

 separate. In maturation and fertihzation (on the right): A and B, the primary 

 spermatocyte and the primary oocyte; C, secondary spermatocyte and secondary 

 oocyte; D, mature egg and three polar bodies, and four sperms; E and F, sperm 

 and egg unite and the nuclei fuse; G, the oosperm nucleus divides; H and /, the 

 cell divides. A-D, maturation stages; E and F, fertihzation; G-/, first 

 cell division. In F-G the two cells are completely fused. Three of the sperms 

 and the three polar bodies are not represented after stage E. 



