374 
For example ova taken from a new set of females were fertilized 
and allowed to develop in water at 12°C. and 22°C.; a few were also 
placed in the sun where they naturally developed very rapidly. 
In each of these three experiments the result was the same, na- 
melv that the third plane of cleavage was Meridional and the 
fourth Equatorial. Itis almost unaccountable that in these latter 
experiments at higher temperatures, no abnormalities were observed, 
which seems to lend strong proof that the mode of cleavage observed 
in P. marinus is the regular one for that species. 
In all of the accompanying figures the first and second planes 
are represented as extending completely around the ovum. 
Fig. I. Fig. II. 
2 
& 
Figures I and II. First (1), Second (2) and Third (3) planes of cleavage. Cam. 
Luc. Zeiss obj. a2, oc. 3. 
As is observed in figures I and II the Third or Meridional 
planes may be variable in regard to their position with respect to the 
first two planes (1 and 2). In some instances they made their appea- 
rance (always in a vertical plane) on each side of and almost paral- 
lel to the first plane; thus dividing the quadrants unequally (Fig. I, 3). 
While in other cases they formed an angle of 45°C. with each of the | 
first two planes, dividing the quadrants into octants of equal size 
(Fig. II, 3). In regard to the relative frequency at which these two 
variations occurred, it can be said that the first appeared more fre- 
quently when the ova were developed at low temperatures, while on | 
the other hand the second variation seemed to be more common among | 
ova developed at high temperatures (22°C.) 
The Fourth or Equatorial plane is situated well up towards | 
the animal pole. It made its first appearance in ova developed in wa- | 
ter at 6°C., 24 hours after fertilization as two furrows situated in each | 
of the two octants nearest the first plane of cleavage (Fig. III, 4a). | 
