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