986 General Notes. [ November, 
with which pigeons feed their young and find that it is nota 
glandular secretion (as is the material with which the Callocalia 
constructs its edible nests), but is a formation of modified epithe- 
lial cells. This material is produced in the cesophagus of both 
the male and the female parents until about the twentieth day 
after the eggs have hatched. 
EMBRYOLOGY.’ 
WHY DO CERTAIN Fish Ova Fioat?—In a recent paper, by a 
Mr. Prince (Ann. and Mag. Nat. History, 1886), his readers are 
informed that the buoyancy of certain fish ova is not due to the 
presence of drops of oil in the yolk as supposed by Ryder, or 
words to that effect. Had my conclusions not been so summa- 
rily disposed of by one whose information is clearly not very 
accurate or extensive, the writer would not trouble himself to 
reconsider the subject of the buoyancy of fish ova. In my Embry- 
ography of Osseous Fishes (p. 118), I have stated that “ the buoy- 
ancy of the cod’s egg is undoubtedly due to the diminished 
specific gravity of the protoplasmic matter of the vitellus, and not 
to the presence of any oils. In this respect it represents an unique 
type of the buoyant ovum.” This statement, published in 1884, 
but written in 1882, is essentially the same as that of Mr. Prince, 
published in 1886. Comment is unnecessary. 
There are several types of buoyant ova. These are: (1) Those 
in which the specific gravity of the yolk is diminished, as in the 
egg of the cod; (2) those in which large oil-drops in an eccen- 
tric position aid in causing the eggs to float; (3) those in which 
a very large oil-drop caused the ovum to float even in fresh 
water. 
These three categories are also, in all probability, connected by 
intermediate kinds; that is, amongst forms of the second series 
there are some which are buoyant in water with as low a 
gravity as 1.014, while others are not buoyant in water of less 
‘specific gravity than 1.025, while those in which the proportion 
of oil to plasma is very great, or about as I to 7, are enh? E 
water with a specific gravity of very nearly 1.000, or in that wh! 
is fre. 
As a rule, the buoyant ovum has the oil gathered into a "e 
drop embedded in the vitellus nearly opposite the germinal a : 
there seem to be few exceptions to this rule. There are also “ 
very few species known which have buoyant ova without eat 
drop, and these are buoyant only in water of rather high Oak 
- gravity. Furthermore, as a rule, fish ova which are buoyant st 
not adhesive, but float about near the surface singly; the ea 
noteworthy exception to this rule is presented by the great pe 
oe _ egg-ribbons of Lophius. 
~ ap Baited by Jonx A. RypeR, Biological Department, Univ. of Penna» Philadel- 
