280 Cell as Basis oj Organic Activity 



EARLY EMBRYOLOGY 



Before beginning its divisions, the zygote is essentially a sphere 

 which nearly always has a gradient of metabolic activity. The region of 

 highest activity is known as the animal pole; that of least, as the vegetal 

 pole. In eggs with a large amount of stored food (yolk), the food is 

 most concentrated at the vegetal pole. 



For the detailed study of the early stages of embryology, the egg of 

 the chordate, Amphioxus, provides an excellent subject. The divisions 

 involve the entire egg and occur at an even rate. The periods which are 

 commonly recognizable in early embryology are cleavage, formation of 

 the blastula, gastrulation, and mesoderm formation. 



Cleavage. — The first divisions of the tgg are termed cleavage. 

 These divisions divide the tgg of Amphioxus into a mass of similar cells 

 known as blastomeres. During this period, the general size of the cell 

 mass becomes little greater than that of the original zygote ; thus the size 

 of the individual blastomeres becomes smaller and smaller as cleavage 

 proceeds. 



The first cleavage plane extends from the animal to the vegetal pole, 

 and splits the egg into two equal blastomeres. The second plane 

 is also from pole to pole, but is at right angles to the first one and results 

 in the 4-celled stage. The third cleavage plane is a horizontal one which 

 results in 8 cells. Sixteen cells are produced by a fourth cleavage plane 

 which occurs again from pole to pole. Following the 16-cell stage, there 

 is an alternation of horizontal and meridional cleavages resulting in 32,- 

 64-, and 128-cell stages. After this the divisions are a bit more irregular. 

 The mass of blastomeres at the time of the 16-cell stage and until ap- 

 proximately the 128-cell stage is known as the morula, due to the resem- 

 blance of the mass of spherical cells to a mulberry. 



Blastula Formation. — Due to the rapid divisions, the cells tend to 

 move out of the solid mass typical of the morula to form a hollow ball, the 

 blastula. This usually occurs at about the 128-cell stage of Amphioxus. 

 The blastula consists of a single layer of cells, the blastoderm, surround- 

 ing a cavity the blastocoel. The blastocoel is filled with a watery jelly. 

 The cells are somewhat larger at the vegetal than at the animal pole of 

 the embryo, and those at the animal pole tend to divide more rapidly 

 following blastula formation. 



