THE GROWTH OF THE EMBRYO IN THE FROG. 
241 
Third change. — In this a third cleft is formed horizontally around the egg, nearly 
midway between the upper and under surfaces : this may be called from its position, 
the equatorial cleft. 
Fourth change. — About two hours after the completion of the crucial cleft, a new 
series of changes is set up in the egg. The clefts no longer include the whole circum- 
ference of the egg, but are confined to the splitting of the larger into smaller pieces 
after a binary plan : and this process does not begin at once over the whole surface, 
but appears first in a given spot, and then pursues a definite course ; thus each of the 
two pieces seen from above on one side (behind) of the crucial cleft become subdi- 
vided, producing four segments on one side of that line, whilst there are only two on 
the other. When this subdivision is nearly completed, and not till then, a correspond- 
ing change takes place in the two segments on the other side (in front) of the sulcus. 
When this stage of the segmentation of the yelk is arrived at, the position of the 
body and head of the coming being can be determined with certainty, so that it is 
not necessary to follow further the changes during segmentation. If the cell be 
marked opposite the first commencing post-crucial subdivisions, and then set aside 
for the formation of the embryo, the trunk and tail of the developing being will be 
found to originate in this first-subdividing part behind the crucial sulcus, and the 
head to be produced in the part on the other side, or in front of the sulcus, in which 
the secondary segmentation last appears. 
On the correspondence of the primary cleft of the Yelk with the axis of the future 
Embryo. 
I have been long aware that the axis of the embryo was in the line of the first cleft 
of the yelk, but my endeavour to show this was not always satisfactory, in conse- 
quence of the difficulty of making the egg keep a given position, whilst it was free to 
move; but since 1 have employed the tube-cell I have obtained the desired evidence 
with great ease. The results of the following observations will support my statement. 
Obs. 1. — I took an egg that had just divided for the first time, and placed it in a 
glass cell only sufficiently large to contain it when the jelly was fully expanded, and 
filled the cell with water. The dorsal surface turned uppermost, as usual, conse- 
quently I had under my eye the whole surface, and could watch the changes with the 
microscope. 1 marked the plate of glass supporting the cell with a line parallel to 
the primary cleft of the yelk, and indicated the position of the ends of the sulcus by 
other marks. The whole was placed in a temperature of 60°Fahr. 
At the time of the closing-in of the dorsal laminae, I found the correspondence 
between the axis of the embryo and the line of the first cleft to be exact. As I knew 
however that some movement would be excited, I made a drawing of the appearances. 
In twelve more hours the dorsal sulcus was nearly closed, and the embryo had 
passed to the left of the line marked on the glass, viz. to about an angle of 30°. A 
second drawing was made to remove doubts. 
2 i 
MDCCCLIV. 
