THE GROWTH OF THE EMBRYO IN THE FROG. 
231 
constructed of brass with glass at the top and bottom, and it is inserted into a brass 
plate by which it may be fixed to the stage of the microscope : the front glass can be 
approximated to or removed from the hinder one by means of the sliding of the piece 
of brass in which it is cemented, though this motion never allows the glasses to touch. 
The glass employed is thin, especially the front one; and a piece is cut out of the 
anterior one (about one-third of the whole) to allow of water being added or removed 
by a siphon without disturbing the box or its contents. Its diameter is eight-tenths 
to nine-tenths of an inch, and its least depth is about one-sixth of an inch, or about 
sufficient for the lodgment of a single row of the frog’s eggs, before they are expanded. 
During the use of this box the microscope is placed horizontally; and a camera is 
attached to the eyepiece to allow of the immediate delineation of the changes seen. 
The magnifying power used has been commonly the half-inch object-glass (Ross) 
with No. 2 eyepiece, but when any doubt has arisen the quarter-inch object-glass has 
been taken ; and the illumination has been derived from the apparatus called Gillet’s 
condenser. 
If the following points are attended to, the entrance of the spermatic body may be 
readily seen. The eggs are to be passed uninjured from the frog*, and are to be 
attached immediately to the inner surface of the glass plate in the moveable front of 
the cistern box; the front of the box is to be replaced and the objects brought into 
focus, and then the box is to be filled with spermatozoic water. As soon as the fluid 
touches the eggs, these imbibe it and expand, but they remain firmly attached to 
the glass. In order that the success of the experiment may be ensured, equal parts 
of the sperm and water may be used, and within a few minutes after the former 
has been obtained ; this will cause a large number of spermatozoa to enter the egg; 
but as the mixture is too opake for the satisfactory observation of the phenomena, I 
remove it at the end of two or three minutes by means of a siphon, and supply its 
place with pure water. By this time many of the sperm bodies have begun to enter 
the ovum, and their transit to the vitelline chamber is facilitated by the endosmose 
of the water. 
When these circumstances have been attended to, spermatozoa may be sometimes 
seen at the zona pellucida of the envelope within the first minute, though only those 
that encounter the egg at right angles; but in from four to five minutes many may 
be visible, according to the number contained in the water. After the lapse of some 
time, varying with the temperature, the formation of the chamber may be noticed 
over the centre of the dark surface of the yelk ; it is usually at this stage, and for a 
short time afterwards, that the spermatozoa are first detected in, or passing into, the 
vitelline chamber. 
Penetration of the Spermatozoon into the Yelk, 
The fact of the penetration was first observed on the 25th of March, 1853, not 
* Philosophical Transactions for 1851, p. 188. 
