112 BULLETIN OF THE 
Apostolides. No change was detected in the ovum of the latter seven 
hours after fecundation. On the seventh hour after the contact of ova 
and sperm, according to Apostolides, the first indications of the segmen- 
tation of the egg appear in Ophiothrix. Seven hours after the capture 
of Ophiopholis some of the ova were found in the 4-cell stage, and a 
little over three hours after the egg was dropped by the female they were 
found in the 2-cell stage. Is this discrepancy the result of a difference 
of temperature in the water ? * 
The second segmentation stage, the 4-cell stage, Pl. I. figs. 6, 7, is 
brought about by a formation of a second cleavage plane, 2 cl pl, at 
right angles to the first. As in the first the two cells of the 2-cell stage 
are separated by a transparent layer, in the same manner each of the two 
cells of the 4-cell stage are divided by a similar plane. A nucleus is 
observed in each of the spheres of the 4-cell stage. The division of the 
2-cell stage into four cells is regular,t and all the cells are of the same 
size up to the 4-cell stage. 
Cleavage. 
The first external change in form of the spherical egg in its segmenta- 
tion is the formation of the first cleavage-plane. A constriction or annular 
groove, destined to divide the egg into hemispheres, forms about the egg 
in the same way as in other Echinoderms. It was not observed whether 
a collar-like { extension of the superficial portion of the yolk sinks into 
the denser central region, or whether the two cells form in some other 
way, but in a well-formed 2-cell stage the two cells are separated by a 
transparent wall. The transparent wall is identical with that which 
covers the denser part of the ovum in its undivided condition. 
The 4-cell stage is formed from the 2-cell stage by a cleavage-plane, 
2 cl pl, at right angles to the first. As in the 2-cell stage the two cells 
or blastomeres are separated by a transparent layer, so in the 4-cell stage 
* Metschnikoff (Zeit. f. Wiss. Zodl, XLII., p. 665) has recorded a great differ- 
ence in the time occupied by the early development of the eggs of the same Echino- 
derm from two localities. These differences are probably due to temperature, 
The influence of temperature on the rate of development of the ova of Echino- 
derms is a subject which would repay an extended investigation. 
+ If any irregularity in size exists the difference is very small. 
t It may be supposed that the superficial layer following a constriction of 
the denser part of the ovum, bisects the latter by a centripetal growth. This 
would correspond with the mode of formation of the 2-cell stage in other Echino- 
derms. 
