INTRODUCTION TO THE METAZOA 



81 



esting process of fertilization. The sperm 

 sometimes enters the egg before the polar 

 bodies are formed, and sometimes after- 

 ward. The sperm brings into the egg a 

 nucleus, a centrosome, and a very small 

 amount of cytoplasm. A mitotic figure soon 

 develops and moves toward the center of the 

 egg. The egg nucleus also moves in this di- 

 rection, and finally both the male and fe- 

 male nuclei are brought together in the 

 midst of the spindle produced about the 

 sperm nucleus. Their union forms the 

 zygote nucleus. This completes the process 

 usually known as fertilization. In this proc- 

 ess the most important result appears to be 

 the union of two nuclei, one of maternal 

 origin, the other of paternal origin. 



Chromosome reduction 



It is now possible to point out the result 

 of the reduction in the number of chromo- 

 somes which takes place during maturation. 

 It has already been stated that every species 

 of animal has a definite number of chromo- 

 somes in its somatic cells, two of each kind. 

 This number remains constant, generation 

 after generation. Now, if the mature egg con- 

 tained this somatic number of chromosomes 

 and the sperm brought into it a like number, 

 the animal which developed from the fer- 

 tilized egg would possess in its somatic cells 

 twice as many chromosomes as its parents. 

 However, the number is kept constant by 

 reduction during the maturation divisions 

 (Fig. 404), so that both egg and sperm con- 

 tain only one-half the number of the somatic 

 cells and primordial germ cells. The union 

 of egg and sperm again establishes the nor- 

 mal number of chromosomes possessed by 

 the parents. 



Embryo, embryogeny, and 

 embryology 



An embryo is a young animal that passes 

 its developmental stages within the egg or 

 within the mother's uterus. Embryogeny is 



the study of the development of particular 

 organisms. Embryology is that branch of 

 biology which deals with the development of 

 an embr}'0. 



Cleavage 



The division of the fertilized egg (zygote) 

 into a number of cells (blastomeres) is 

 known as cleavage. The chromatin material 

 in the zygote nucleus becomes organized 

 into chromosomes. Each chromosome dupli- 

 cates itself; these daughter chromosomes are 

 so arranged on the first cleavage spindle that 

 each daughter nucleus receives either the 

 original or the duplicate of each chromo- 

 some. After nuclear division, comes the di- 

 vision of the zygote into two blastomeres. 

 This means that each blastomere will re- 

 ceive one of each chromosome of parental 

 origin, and one of each chromosome of ma- 

 ternal origin. Further divisions insure a like 

 distribution to every cell of the body. The 

 blastomeres do not separate, as do the 

 daughter cells produced by the binary divi- 

 sion of the Paramecium, but remain at- 

 tached to one another. The resemblance of 

 the group of blastomeres to a mulberr\' sug- 

 gests the term morula which is sometimes 

 used in describing the egg during the early 

 cleavage stages. 



Several types of cleavage patterns are rec- 

 ognizable. If the eggs contain relatively little 

 yolk, the entire zygote divides into 2, 4, 8, 

 etc., blastomeres. If these daughter cells are 

 approximately equal in size, the process is 

 known as equal holoblastic (total) cleavage. 

 Such a cleavage pattern is characteristic of 

 the eggs of starfish and amphioxus (Fig. 41 ) . 

 If the daughter cells are of unequal size, the 

 process is known as unequal holoblastic 

 (total) cleavage; this cleavage pattern is il- 

 lustrated by the frog egg except during the 

 first two or three cleavages (Figs. 41 and 

 237). If the eggs contain a considerable 

 amount of yolk, the entire egg is not divided 

 into cells; only restricted portions of the 

 cytoplasm undergo cleavage. If cell divi- 

 sion is restricted to a small cap or disk on 



