CLEAVAGE IN METAZOA 77 



germ cells. Furthermore, the first two division planes are ver- 

 tical and the third horizontal, exactly as in the case of holoblas- 

 tic cleavage in metazoa. 



The primordial cell (fertilized egg cell) of a higher animal 

 develops by continual cell division until myriads of cells con- 

 stituting the adult organism are formed. In the typical case 

 and in the majority of forms the early stages of this development 

 follow the course illustrated in Fig. 33. The process of cleav- 

 age, so-called, being a regular and symmetrical division of the egg 

 cell. The first cleavage in a vertical plane results in the for- 

 mation of two similar cells the two-cell stage; the second' 

 cleavage, also vertical, gives four similar cells; the third cleav- 

 age differs in being horizontal, crossing the first two planes at 

 right angles and resulting in eight cells; in the majority of cases 

 this third cleavage brings about the first trace of differentiation 

 in the cells, those of one pole (vegetative), being larger and con- 

 taining more yolk than those of the smaller pole (animal) . The 

 fourth cleavage is again vertical and the difference between the 

 two poles is further emphasized, the sixteen cell stage present- 

 ing eight larger vegetative and eight smaller animal cells. At 

 this stage also the cells begin to separate leaving a cavity in 

 the center. This cavity, termed the segmentation cavity, in 

 the majority of types is later closed up and plays no part in the 

 adult organism. After the sixteen cell stage cleavage becomes 

 more or less irregular, the cells of the animal pole dividing more 

 rapidly tha-n those of the vegetative, until a many celled hollow 

 sphere results. At this stage the organism is termed a blastula 

 (Fig. 33). Up to this time the developing metazoon differs but 

 little from some colony forms of protozoa. After the blastula 

 stage a step in development is taken which is found nowhere but 

 in metazoa and which represents therefore a great advance 

 over protozoa. The cells of the lower pole invaginate or turn 

 in until their inner sides come in contact with the inner sides of 

 the cells of the animal pole. This process is termed gastrulation. 

 The gastrula thus formed being a double walled sack enclosing a 

 cavity which becomes the enteric or digestive cavity of the 

 embryo, and in most cases the digestive tract of the adult is 



