434 
DE. W. H. EANSOM ON THE OVUM OF OSSEOUS FISHES. 
micropyle, and at its margins passing imperceptibly into the thinner layer of similar 
material which extends over the rest of the yelk (Plate XV. fig. 5, and Diagram A). 
Although this layer is found in the eggs of both species of Gasterosteus with iden- 
tical structure and properties, it is best observed in those of G. pungitius. 
It may be traced over the whole surface by examining the eggs under gentle pressure, 
either rolling over the same egg, or using at the same time a number of eggs in different 
positions, or by treating an egg with a weak solution of acetic acid, which so hardens the 
cortical layer that it cracks under pressure, and is then very distinct, even at the ventral 
pole, where it is thinnest. To show the thicker portion of it at the germinal pole, or 
discus proligerus, it may be viewed either full face under gentle pressure short of rup- 
ture, when it is seen as an opaque, yellowish halo around the micropyle ; or in profile, 
when it is seen as in Plate XV. figs. 5, 6 & 8. 
A power of x50 enables one familiar with the object to trace this layer very well, 
but to make out its constituent parts X 100 is required, and then it is better to examine 
both before and during rupture, under suitably graduated pressure. By such means 
it may be made out to contain a number of droplets, sometimes irregular in form from 
mutual pressure, but usually round, and varying much in size from a diameter equal to 
the thickness of the yelk-sac down to an immeasurable granule, in which case the cha- 
racters of a drop are lost. They have a yellowish colour, are placed near the surface of 
the egg next to the inner sac, and the larger ones are much more numerous in the ger- 
minal than in the ventral segment. They are imbedded in a mass of fine, yellowish, 
granular matter, which in part at least consists of very minute similar droplets, but 
principally of a substance having somewhat different reactions. 
These elements are held together by a homogeneous matrix, which in the discus 
proligerus makes the mass semi-solid, and under some conditions may be seen drawn 
into threads, as if very viscid. 
In consequence of the presence of the granular elements of the formative yelk in the 
cortical layer of the unimpregnated eggs, they are more opaque to the naked eye than 
impregnated ones. 
Attached to the basal surface of the discus proligerus, in contact with the clear food- 
yelk, there is a small collection of dark oil-granules, distinct from the larger drops 
which float in the food-yelk. 
The yellow droplets are characterized by their reactions with water, and indeed they 
are so unstable that I could find no neutral medium in which to examine them when 
the egg is ruptured. When seen in situ in unimpregnated, unruptured eggs, into which 
no water has been imbibed, in consequence of the defensive action of the viscid layer, 
their aspect is perfectly homogeneous and highly refractive, although less so than oil. 
On rupturing an egg in water, they exhibit vacuolation very rapidly, and undergo a very 
varied series of changes, during which they become pale and disappear, often presenting 
appearances like cells, with clear, lilac-tinted, vesicular nuclei in a mass of a deeper 
yellow colour, either granular or homogeneous. Similar changes occur, but less rapidly, 
