EMBRYOLOGY OF THE SEA BASS. 
211 
hij-er of protoplasm wliicli is coiitiiuied rouiul the yolk. The yolk itself contains no 
; protoplasm except in the immediate neighborhood of the oil drop. About a third of 
the surface of the latter is covered by a cap of coarsely reticulated protoplasm, lacking a 
i i nucleus. Fig. 1 shows a section through the oil globule, o. //., and its protoiilasmic 
cap, 0 . (j. which is entirely free from the superlicial (almost imperceptible) layer of 
nrotoplasm. This cap of coarse protoplasm is easily seen in surface views, and 1 have 
observed it iu Mackerel eggs as well as in those of the Bass. 
The patch of protoplasm at the lower pole, or the biastodisc, is at first circular. 
Just before or during the first act of cleavage there arises aii inequality of the axes, 
so that by tlie time the first two blastomeres are marked ol'f, the germ is bilateral, or 
at least biradial (Fig. 2). In the Bass and Mackerel the first two blastomeres are of 
s equal size. This is normally so with the Cod as well, but ou one occasion I observed 
that iu all of the eggs got from a single codfish, the first two blastomeres were un- 
equal iu size. The inequality was very marked, but the eggs were healthy and the 
' average percentage offish was hatched out. 
II The first plane of cleavage is claimed bj^ Agassiz aud Whitman (1) to represent 
j the anteroposterior axis of the adult. I found that while I could follow with ease the 
I succession of furrows until thirty-two cells were formed, and in some blastoderms 
' could follow the process a step further until sixty-four cells were established, after 
i I that I could no longer trace the fate of tlie early cleavage planes. 
The segmentation of the Bass is of the ordinary bilateral type characteristic of 
Teleostei. The first two planes of cleavage (Fig. 2) are meridional, and at right angles 
to each other. Tliey cut very deeply into the biastodisc, but do not extend quite 
through to the yolk. The section (PI. lxxxix. Fig. 13) really belongs to a stage of four 
; blastomeres, but does not differ in appearance from one through the two blastomere 
stage. As is seen in this section, the two segments and later the four are connected 
i by a thin layer of protoplasm {c. p., central ])eriblast), in the center of the bla.stodisc. 
I At The periphery the segments are continued into the superficial protoplasm clothing 
■ the yolk, which protoplasm is especially thickened round the immediate edge of the 
I segments. The ridge thus formed (early periblastic ridge, e. p. r., Figs. 2, 8, etc.) per- 
;j sists from the time when cleavage begins until the periblastic nuclei are established 
f as such (Fig. 22). During all this time it constantly varies in distinctness, now being i 
' very obvious and again scarcely jierceptible. It has long been known that the blas- 
1 todisc increases in size during segmentation, and the lluctuation in the height of the 
ridge is undoubtedly due to some periodicity in the force, which effects the incorpo- 
ration of the outlying protoi)lasm into the biastodisc. Agassiz and Whitman (1. c., p. 
' 49) discuss this point and conclude that the force is in some way associated with 
I nuclear division. The ridge, which I have called the ‘‘early periblastic ridge,” to dis- 
tinguish it from the peripheral periblast wall which is formed in a much later stage 
by the fusion of certain of the blastoderm cells, is shown in section in Figs. 15, 16, 
I PL LXXXIX, Fig. 17, PI. xo. In Fig. 13. PI. lxxxix, and Fig. 14, PI. lxxxix, the 
ij ridge is at its lowest ebb, so to speak, and the blastomeres fade away gradually into 
( the surrounding protoplasm. 
The third cleavage plane is shown in Fig. 3. It is in most eggs nearly, and often 
quite, parallel to the first plane. In this stage tlie segmentation cavity, s. c., which is 
: perfectly distinct after another cell division, becomes recognizable. Figs. 14 and 15, 
) PI. LXXXIX, are two sections through the planes a and 5, respectively, of Fig. 3. In the 
