EMBRYOLOGY OF THE SEA BASS. 
213 
t isome niiusually symmetrical embryos, it is possible api)roximately to deduce an ideal 
[type, which, it may safely be said, is never exactly followed in actual segmentation. In 
Fig. 1-5 it is seen that the cells of the peripheral ring undergo either meridional or 
equatorial division, but it is impossible to decide from the relative frequency which 
I should be regarded as the type. 
Figs. 18 and 10 are two sections through unusually symmetrical germs, in which 
the thirty-two cells had nearly retained the positions indicated by the amphiasters of 
Fig. 8. They may be referred to the ideally symmetrical thirty-two cell stage, as repre- 
j seated by Fig. 1. Fig. 18 would lie in the plane a-b, and Fig. 10 in the plane d-f. In 
Fig. 18 the lower tier (four in all) 
of central cells, c-z, are in the first 
stages of nuclear division. The 
.cleavage planes of these four 
cells, then, run in the direction 
. ir. Fig. 1. Fig. 19 shows that the 
5 four upper central cells (oi) suffer 
cleavage in planes i>arallel to t/. 
.These planes, x and y, may be 
I taken as typical for the central 
I cells, though there is undoubt- 
'edly much variation, but never in 
the direction of horizontal cleav- (2/ 
age. The horizontal cleavage is 
exclusivelj^ found in the cells 
which lie between the central 
: cells and the peripheral ling, viz, 
dn W 2 , n-z, etc. In Fig. 10 two of 
'these cells, niz and mz, show this 
:! cleavage. 
The typical cleavage from 
thirty-two cells into sixty-four 
‘may then be described as fol- 
lows: The peripheral cells suffer meridional (equatorial) division, the intermediate 
cells, m-z, iiz^ etc., horizontal division, the four upper central cells and the four lower 
central cells divide in planes parallel to y and x, respectively, these planes, being, in 
i all probability, metamorphosed meridional planes. The further course of segmentation 
I found it impossible to follow. Fig. 20, PI. xc, is through a blastoderm about an 
hour older than Fig. 19. The peripheral cells contkiue to divide meridionally or equa- 
torially, the plane obviously being decided in many cases by the greater length of 
one of the cell axes. 
Duringthe segmentation the periodsof rest are, as usual, long compared with those 
of activity. I give the following time record of the early divisions : 
11:40 to 11:58 (18 minutes) period of rest; 4 blastomere stage. 
11:58 to 12 (2 minutes) i^eriod of activity ; 4 into 8 blastomeres. 
12 to 12:27 (27 minutes) period of rest; 8 blastomere stage. 
12:27 to 12:30 (3 minutes) period of activity; 8 into 16 blastomeres. 
Bilaterality of the blastoderm . — Occasionally a blastoderm is met with which sug- 
gests that the bilaterality in at least the early stages is very deeply seated indeed. 
/ 
Fig. 1. — Ide.T,lly syninietrical 32-ceIl stage. 
