AN INTRODUCTION TO THE METAZOA 87 



at the surface and form a disc-shaped region (Fig. 50, 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. 50, 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, as follows: (1) 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. 51, A) results in the pro- 

 duction of two (B), four (C, D), eight (E), sixteen (F), etc. 

 cells approximately 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. 40, 

 o-q), but remain attached to one another. The resemblance 

 of the group of blastomeres to a mulberry suggested the term 

 morula, which is often used in describing the egg during the 

 early cleavage stages. 



Blastula. — As cleavage advances, a cavity becomes notice- 

 able in the center of the egg (Fig. 51, H) enlarging as develop- 

 ment proceeds until the whole resembles a hollow rubber 

 ball, the rubber being represented by a single layer of cells. 

 At this stage the egg is called a blastula, the cavity the cleavage 

 or segmentation cavity, and the cellular layer the blastoderm. 

 The blastula resembles somewhat a single colony of Volvox 

 (Fig. 27). 



Gastrula. — The cells on one side of the blastula are seen 

 to be thicker than elsewhere (Fig. 51, K) and begin to in vagi - 

 nate (Fig. 51, L). This process results in a cup- shaped struc- 

 ture with a wall of two layers, an outer layer of small cells and 

 an inner layer of larger cells. The embryo may now be called 

 a gastrula (M) , and the process by which it developed from the 



