THE MICROCOSM OF THE EGG 135 



mammals, where a common size is one-tenth of a 

 millimeter, say a tenth of the diameter of an average 

 pin's-head. In the large eggs of birds, reptiles, 

 sharks, and the like, the bigness is due to the accu- 

 mulation of a huge capital of yolk. On the top of 

 this there lies a drop of formative living matter, 

 like a miniature watch-glass turned upside down. 

 The egg-cell of a whale, containing the potentiality 

 of the colossal creature, is no larger than fern 

 seed; and it is worth noticing that the male-cell 

 or sperm-cell is, according to the kind of animal, 

 several thousand times smaller than the egg-cell, 

 and thousands of sperms may rush around in a drop 

 suspended from the head of a pin. 



In spite of its minuteness, the egg-cell contains 

 many different kinds of components, which are 

 often, if not always, disposed in zones or after 

 some specific pattern. According to one school, 

 the regions of the egg differ qualitatively, some 

 having particular "organ-forming substances" 

 which others have not; according to another 

 school, the regions differ quantitatively in the 

 degree of concentration and in the rate of reaction 

 of the constituent formative materials. Probably 

 the truth is with both schools. Delicate experiment 

 has made it certain that in some egg-cells, e.g. those 

 of sea-squirts (Tunicates) and sea-gooseberries 

 (Ctenophores), particular areas of the egg, some- 

 times distinguishable by their color, will normally 

 develop into particular organs of the animal. On 

 {he other hand, the pattern of most egg-cells is not 



