246 BULLETIN OF THE 
Cleavage. 
First Cleavage Furrow. — Gegenbaur,* who says that he observed the 
segmentation of the ovum of the genera Agalmopsis, Physophora, Fors- 
kalia, Hippopodius, and Diphyes, states that the whole process of seg- 
mentation is finished in from twenty-four to thirty-six hours. Haeckelf 
says that in Physophora, Crystallodes, and Athorybia the segmentation 
is finished at the end of the second day. Metschnikoff does not state 
the exact limit of time when the segmentation is finished, although 
from the age of the youngest larva of Aga/ma which he figures I should 
judge that the segmentation was accomplished in the second day. All 
recorded observations on Siphonophore eggs point to the conclusion that 
the cleavage is wholly completed before the beginning of the third day 
after fecundation. 
My first specimen of Agalma was captured on August 6th, at 
noon, and before the morning of August 8th it had laid eggs which 
were in the same stage as that figured by Metschnikoff on the fourth 
day. In other words, a little over a day and a half after the Agal- 
mata were placed in the aquarium, eggs from them had segmented and 
had formed the two layers described by Metschnikoff in the changes 
of the fourth day. My observations are thus at variance with those of 
Gegenbaur, Haeckel, and Metschnikoff. What is the meaning of the 
discrepancy? Looking over my notes in vain to find an error in this 
particular, it has seemed possible that errors of observation have crept in 
for the reason that individual eggs have not been followed through their 
consequent stages. An Agalma in captivity will mature its eggs at dif- 
ferent times, so that at the end of the fifth day segmented eggs in com- 
pany with those which are far along in the development of the primitive 
hydrophyllium may be picked out of the same water. From the nature 
of the case, unless individual eggs are isolated and the time of their 
fecundation recorded, it is impossible to know the age of any specified 
stage. 
The first change which takes place in the spherical egg after it has 
left the gonophore is the formation of the primary cleavage furrow, pr. 
At one pole of the ovum (PI.I. fig. 7) an indentation appears in the 
form of a furrow on the surface of the egg. Although I have not ob- 
served at the outset the exact relationship of this furrow to the rosy 
pole, I have seen that later, after the first plane of cleavage has been 
* Op. cit., p. 50. 
+ Op. cit., for Physophora, p. 19 ; for Crystallodes, p. 51; for Athorybia, p. 89. 
