108 AN INTRODUCTION TO ZOOLOGY 



maternal origin. Further mitotic divisions insure a like distribu- 

 tion to every cell in the body. After nuclear division comes the 

 division of the entire cells into two (k and /). 



Typically the fertilized egg divides into two cells, these two 

 into four, these four into eight, etc., each cleavage plane being 

 perpendicular to the last preceding plane (Fig. 53). This is 

 known as total cleavage, and is characteristic of holoblastic eggs. 

 Other eggs are said to be meroblastic because they exhibit partial 

 cleavage, that is, only a small part of the egg enters into cell divi- 

 sion, the remainder serving as nutritive material for the cleavage 

 cells. In all we can recognize four distinct types of cleavage: 

 (i) equal cleavage, where the egg divides into two equal halves 

 (Fig. 52, A); (2) unequal cleavage, where the first division of the 

 egg results in one large and one small cell (Fig. 52, B); (3) dis- 

 coidal cleavage, where the entire egg does not divide, but small 

 cells are cut off at the surface and form a disk-shaped region 

 (Fig. 52, C); and (4) superficial cleavage, where the nucleus of the 

 egg divides rapidly; the daughter nuclei migrate to the periphery 

 and form a single layer of cells at the surface (Fig. 52, D). 



That part of ontogeny which concerns the development of an 

 animal from the egg to maturity is known as embryogeny. Cer- 

 tain stages in this development have been recognized as common 

 to all higher animals, and have been given names. The stages 

 occur in a certain regular order, and as an introduction to the more 

 detailed special accounts given for each type studied in subse- 

 quent chapters, we shall present a brief embryological history of a 

 typical holoblastic egg. The stages to be considered are: (i) 

 cleavage, (2) the morula, (3) the blastula, (4) the gastrula, (5) the 

 formation of germ layers, and (6) organogeny. 



Cleavage in a holoblastic egg (Fig. 53) results in the production 

 of two (B), four (C, D), eight (E), sixteen (F), etc., cells approxi- 

 mately equal to one another and growing smaller as their number 

 increases. Each of these cells is known as a blastomere. The 

 blastomeres do not separate as do the daughter cells produced 

 by the binary division of Paramecium (Fig. 31, o-q) but remain 



