



Fig. 26. Fertilization (after Wilson). (I) Sperm contacts egg. (2) Sperm 

 head and midpiece enter egg through fertilization cone; egg nucleus pro- 

 ceeds through meiosis II. (3) Sperm nucleus enlarges; midpiece constructs 

 spindle and asters; meiosis II of egg completes to yield polar body and 

 functional, haplold egg nucleus. (4) Sperm and egg nuclei migrate toward 

 each other across midpiece spindle; nuclear membranes break down. (5) 

 Chromosomes dispose themselves upon mitotic spindle. (6) Chromosomes 

 duplicate and mitosis draws to a close in preparation for first cleavage. 



phase of mitosis. The chromosomes duphcate, the first cleavage of cells 

 occurs, and two daughter cells, now diploid, emerge. The egg has become 

 an embryo. 



Cleavage 



In all eggs, fertilization is followed by a period of extensive cell divi- 

 sion without an increase in size. The fertilized egg is cleaved repeatedly 

 into smaller and smaller cells. The early divisions are generally synchro- 

 nous; thus the egg yields two cells, the two produce four, the four, eight, 

 and so on. From this time on, synchrony may disappear and different parts 

 of the embryo may cleave at different rates. At the end of the cleavage 

 stage, the embryo, now called a blastula, is a ball of thousands of small 

 cells (blastomeres). 



Although there is no size increase during this period, a considerable 

 amount of synthesis occurs. The cleavages are regular mitotic divisions so 

 that chromosomes and total nuclear content must be duplicated in the 

 daughter cells. This means extensive synthesis of deoxyribose nucleic 

 acid plus other nuclear constituents. In addition, a great deal of ribose 

 nucleic acid and proteins, including enzymes, are formed in the cytoplasm. 

 The energy and raw materials for these processes come, of course, from 

 the supply of yolk. 



54 



