158 THE MICROSCOPICAL NEWS. 
Frog spawn, as you are aware, is not to be found all the year 
round ; the time to find it is from about the first week of March 
till about the middle of April, according to the severity or other- 
wise of the weather, deposited most frequently at the shallow 
sides of ponds or ditches amongst the grass or aquatic plants ; 
floating in gelatinous masses not unlike bunches of white currants, 
or, as some one has put it, a tapioca pudding, in which the yolks 
of the eggs are seen as round black specks about >; of an inch in 
diameter. When the eggs are deposited the outer or gelatinous 
envelope takes up a large amount of water, and swells to many 
times its original size into a transparent sphere about 7% of an 
inch in diameter. If we examine the egg it will be seen to contain 
a supply of numerous little globules of air, which, besides supplying 
embryo tadpoles, give to the egg more buoyancy by converting it 
into a sort of water balloon. 
Some may be disposed to ask the question, What sight of interest 
is to be found in the egg of a frog? My answer to this is, that the 
changes seen in the frog’s egg are in the highest sense of the word 
interesting, because in these changes we are initiated into the 
secret of nature’s first steps in its manufacture of a living form. 
And, further, that as far as science or research has gone, the 
changes seen in the egg of the frog are found to be common to 
the entire animal world. 
In the first stage of development the yolk of the egg is seen 
undergoing a process of segmentation or division ; this division 
proceeds with the utmost regularity, until, in the end, it is seen 
to have divided into an immense number of cells, so closely 
packed as to resemble a mulberry. Thus the concluding stage of 
all egg segmentation is aptly named the mulberry stage. 
In those changes in the egg of the frog we have exhibited to the 
observer that which is common to animal development at large. 
This process of segmentation can only be followed by the destruc- 
tion of an egg at regular intervals, either by crushing it between 
two glass slips, and examining it in water under a % or ¥% inch 
objective, or by cutting a section. By either of these processes 
the segmentation can be followed through all its stages. Exter- 
nally the appearance of the yolk changes little till about the end 
of the fourth day, when it begins to assumé an oval shape, until at 
the end of the fifth day a groove is run across this oval, dividing it 
into two parts. 
This is a most interesting and important period in the develop- 
ment of the embryo, as at this time the formation of two mem- 
branes is effected, and by their development the young embryo is 
to be formed. ‘This primitive groove is formed in that part of the 
body which is to form the back part or region. This groove is 
then contracted into a tube, so as to shut off the other parts 
