466 
DE. W. II. EANSOM ON THE OVUM OF OSSEOUS FISHES. 
yelk-sac, and the inner sac had shrunk to a variable amount. Over the partly emptied 
inner sac, where it still contained some food-yelk, a wave of contraction was sometimes 
seen to pass slowly, but distinctly and repeatedly. The observation was made in several 
eggs. 
Thus the contractions of the yelk in Gasterosteus are independent of impregnation, 
although ordinarily they are only seen in fecundated eggs, and they may continue long 
after all vital processes might be supposed to have ceased, and while all around, and in 
contact with the contracting matter, is decomposing. 
b. The cleavage , which is limited to the germinal disk, begins usually about two hours 
after fecundation, although sometimes as early as l h 25 m , or as late as 4 h , varying chiefly 
with the temperature. 
The formative yelk, having been concentrated as described, varies considerably, as 
to its form, at the moment when it is about to cleave. It may be flat and somewhat 
diffused, with its periphery well defined, or not, it may be prominent and conical, or 
hemispherical ; in short those modications of its form which result from the then fading 
remnants of the waves of contraction, are still going on at the moment when a fresh 
set of contractions begin, viz. those which result in cell-formation. Plate XVII. fig. 49 
shows its usual aspect immediately before cleavage, and the moment when the first cleft 
is beginning is shown in Plate XVII. fig. 50. 
Adhering to the under surface of the germinal disk is the group of minute oil-granules, 
which are more numerous at this stage than at an earlier one ; and as there is a constant 
consumption of the stock of oil in the group of larger drops, during the development 
of the germinal mass, it appears probable that at the surface of contact between the two 
kinds of yelk, a digestion of oil, so to speak, goes on ; the process having for one of its 
constant phenomena a subdivision of the oil into minute granules. The remarkable 
appearances which attend the vacuolation of the matter of the formative yelk have 
often, irrespective of the evident absorption of oil into its substance during development, 
led me to infer that it was a compound, containing some fatty substance, easily separable 
from its associated matter. As to its structural elements, the germinal disk differs in 
this stage in no other respect from its earlier condition ; but when crushed and examined 
under higher powers, a few yellow droplets are seen in it, and it is more solid than it was. 
The cleavage begins in a faint well-defined line, which, seen in profile, appears as a 
notch, dividing the germinal disk into two equal halves (Plate XVII. figs. 50 & 52). 
This deepens and gradually separates the germinal disk into two conical elevations 
(Plate XVII. figs. 51 & 53). Even during the cleavage, constantly recurring, very 
slight waves of contraction go on, change the form of the cleavage masses, and cause 
the periphery of the germinal disk to vary. The two first cleavage masses, after the 
stage of greatest separation (Plate XVII. fig. 51), approach each other and appear as 
if about to fuse; this would seem due to the yelk contractions; it is, however, common, 
if not constant, just before the next cleft begins (Plate XVII. fig. 54). 
The inner sac is thrown into folds at the margin of the cleft during its formation, 
