448 GENERAL CONSIDERATIONS 



484. Maturation of the Germ Cells. — In the maturation of a sperm 

 cell is involved its metamorphosis into a mature sperm cell in which a 

 head, middle piece, and tail are recognizable. Egg cells exhibit no 

 metamorphosis but merely accumulate a supply of yolk. This has been 

 described in connection with the origin of the sex cells in Chap. XXIII. 

 The steps there given are true of animals generally. There is, however, 

 one modification known as tetrad formation to which no previous reference 

 has been made. In synapsis it is sometimes true that not two chromo- 

 somes are seen but that a group of four appears. This has been shown 

 to be due to a precocious division of the two chromosomes of a synaptic 

 pair. In this case the four are known as a tetrad; and its division into 

 two bodies, each composed of two chromosomes, one derived from one 

 parent and one from the other, produces a dyad. This is equivalent to a 

 mitosis, and chromosome reduction occurs when the two of the dyad 

 become separated in the second maturation division. 



485. Egg. — In the maturation of the egg cell is usually produced a 

 protective cell membrane which, with the egg cell, forms the egg. In the 

 higher vertebrates a variety of protective coverings may also be added, 

 until in the case of the reptiles and birds an egg becomes composed of 

 added material which may exceed in bulk the egg cell itself. In the case 

 of the eggs of many invertebrates which possess a shell, particularly those 

 of the arthropods, there is an opening in the shell known as a micropyle 

 through which the sperm cell enters in fertilization. In the case of 

 vertebrates, however, the protective envelopes which are added prevent 

 the entrance of a sperm cell, so fertilization occurs before their addition. 

 In the case of reptiles and birds, indeed, so much time elapses between 

 fertilization and the laying of the egg that the egg, when it is laid, contains 

 an embryo and not an egg cell. 



486. Fertilization. — The time and place of fertilization vary in differ- 

 ent animals. The egg may be fertilized while still within the body of the 

 female, in which case fertilization may take place at any point along 

 the oviduct; or it may not occur until after the egg is laid. In some 

 cases the sperm cell enters the cytoplasm of the egg cell before maturation 

 is complete, in which case the union of the two pronuclei (Sec. 139) is 

 delayed until the formation of the second polar body. Sometimes, indeed, 

 the entrance of the sperm cell is necessary to bring about the maturation 

 of the egg cell. 



487. Cleavage. — Soon after fertilization — in some cases within a few 

 minutes — cleavage occurs and the development of an embryo is begun. 

 In Chap. XXV attention was called to differences in the distribution of 

 the yolk in homolecithal, telolecithal, and centrolecithal eggs. This 

 yolk distribution also affects the cleavage. Homolecithal eggs are holo- 

 blastic and if there is only a small amount of yolk and it is quite evenly 

 distributed, cleave totally and equally; if there is more yolk and it is not 



