4 S. F. CLARKE. 



If a freslily laid egg l>e stri|)pecl entirely of its shells and all 

 adhering gelatinous matter, it will be found to be divided into two 

 zones which are almost exact hemispheres, marked out by colors. 

 One hemisphere is black, and the other quite liglit, almost white. 

 The light portion is not evenly colored; the lightest part of it 

 forms a zone lying next to the dark hen)isphere and the darkest 

 portion of the light hemisphere is at the pole, the spot where the 

 vitelline plug is to be formed. This coloration changes as devel- 

 opment goes on. Although the lighter hemispliere is not a clear 

 white, it is a sufficiently light color to make the two heraispiieres 

 quite sharply defined. It can readily be seen by pricking open an 

 egg and allowing the contents to flow out, that this coloring 

 matter lies on the surface, the inner contents of the egg being 

 uniformly opaque — white. If one of these unfertilized eggs be 

 placed in water, it instantly and always assumes a position with 

 the dark hemisphere up and the light pole down ; and as often as 

 it is turned over in any other position it immediately rights itself 

 when the retaining force is removed. As sections show no cavity 

 in the eggs at this period it must be that the density of the 

 unfertilized egg is not uniform and that the lighter colored is 

 always the denser hemisphere. The cavity of Von Baer has not 

 vet appeared, and that moreover would not, probably, be large 

 enougii to cause such a difference in density as is Indicated by the 

 quickness with which the unrestrained egg always takes this posi- 

 tion. After the segmentation-cavity is formed, that portion of 

 the spherical yolk containing this cavity is of course lightest and 

 uppermost; but before segmentation and fertilization, when no 

 cavity exists, this action must be produced by a difference in 

 density of the particles composing the yolk. What can be the 

 object of this arrangement by which the different colored poles 

 are thus placed, it is difficult to conjecture. The darker col- 

 ored areas would absorb more heat from the sun's rays, which 

 under the usual natural conditions would be beneficial to ra[)id 

 development. The arrangement is the same also as the protec- 

 tive coloring in many birds and fishes; the upper side dark and 

 the under side light. This might be of some service to them, as 

 fish of large size might eat small bunches of eggs and would 

 attack them from below, as the egg-masses are usually at or 

 near the surface. Goette says in respect to this coloring of Ba- 

 trachiau eggs — 



