28 The Microscope. 



the resulting cells corresponding to the right side of the body, 

 the other to the left. Whether this is true of all forms has yet 

 to be determined. Attention has only recently been called to 

 the subject, and besides, the investigation is not easy in prac- 

 tice — facts which account for our comparative ignorance on the 

 subject. The second furrow crosses the first at right angles, 

 and in eggs with an equal segmentation these first two planes 

 of division correspond to the meridians of the globe, while the 

 next one, the third, represents the equator. Until the third 

 division the segmentation is much the same in all forms, but 

 here variation becomes marked, and in succeeding stages the 

 differences become increased. 



When an egg has no food yolk, or when this element is dis- 

 tributed equally throughout the egg, the equatorial furrow 

 occupies a median position between the poles, just like the 

 equator on the globe ; but if there is more protoplasm at one 

 end than at the other, the equator is removed towards the active 

 pole, the amount of change being regulated by the amount of 

 protoplasm aggregated at the other. 



Thus, as is apparent, a large amount of food yolk has a 

 very important influence on the character and regularity of 

 segmentation, and in some cases (as in most molluscs) these 

 differences between the cells at the two poles of the egg are 

 very great, so that the smaller ones are called micromeres, the 

 larger macromeres. By this early difference in size the cells 

 may be divided into two categories, a fact which in the typical 

 segmentation does not occur until a much later date. Having 

 now an idea of a typical segmentation, and of the causes which 

 are productive of irregularity, we may proceed to the defini- 

 tions of a few terms, and leave the more peculiar types of seg- 

 mentation to be described in connection with the forms in 

 which they occur. 



The planes which divide up the original egg into cells are 

 known as segmentation planes, and the resulting cells, before 

 they become differentiated into the germinal layers (to be men- 

 tioned further on), are known as blastomeres. The layer of 

 blastomeres forms what is known as the blastoderm, a term 

 usually restricted to the cellular portion in eggs with a large 

 amount of food yolk (like the egg of the common fowl). In 



