ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 11 
vesicle contains an abundant granular precipitate, an enormous nucleolus, and at 
wide intervals within the vesiele, but chiefly near the nuclear membrane, a few 
deeply staining chromatic granules. These granules are small at this time and it is 
difficult to determine their exact shape, though many of them appear to be V- or 
Y-shaped; they are the bivalent chromosomes of the first maturation division. 
Close around the germinal vesicle and extending out nearly to the periphery of the 
egg 1s the yolk, whieh exists in the form of spherules, imbedded close together in 
the granular eytoplasm. Finally there is the peripheral layer of deeply staining 
protoplasm in which the test cells were formerly imbedded and which contains no 
yolk, but numerous refractive spherules much smaller than those of the yolk. 
In the living eggs of Cynthra this peripheral layer is clear and transparent and 
contains uniformly but sparsely distributed yellow pigment, which seems to be asso- 
ciated with these small refractive spherules. "This pigment is soluble in alcohol 
and hence cannot be observed in fixed and prepared material; on the other hand, 
the alcohol in which large numbers of these eggs have been preserved, has the color 
of a solution of potassium bichromate. The test cells of CyzAza also contain yel- 
low pigment granules which are gathered close around the nuclei of these cells. It 
is noticeable that most of the viscera of Cynthza contain this same yellow or orange 
pigment, the ovaries being especially highly colored. This pigment is much denser 
in some individuals than in others, and correspondingly one finds some ova in which 
there is little or none of the pigment, while in others it is very abundant. In gen- 
eral the animals which have little of the pigment in their viscera are those which 
produce eggs with little or no pigment, while those in which the viscera are deeply 
pigmented produce well-pigmented eggs. Тһе central yolk mass of the living egg 
of Cynthia is of a slaty gray color, while the germinal vesicle is clear and trans- 
parent. "Therefore, in the living egg of this species of ascidian, three areas can be 
distinguished with great clearness before the maturation divisions begin,—the 
peripheral layer of protoplasm containing the yellow pigment, the central mass 
of gray yolk and the clear germinal vesicle. 
In Суола and Molgula also these three areas are distinguishable in the living 
egg before maturation, but not so clearly as in Cyzthza. In Crona the peripheral 
layer is nearly transparent, the yolk is a brownish red, while the germinal vesicle 
is also transparent. In Molgula both the peripheral layer and the germinal vesicle 
are transparent, while the yolk is gray, with a faint Шас tinge. А brief. inspection 
of the eggs of Boltenza shows that in this genus the yolk is a bright red. 
This peripheral layer of protoplasm, which is present in all the ascidian ova 
which I have studied, is, both in living and in stained material, the most striking 
feature of the egg before maturation and fertilization. It is surprising therefore 
that in spite of this fact it has received so little attention from those who have 
studied the ovarian history of the ascidian egg; in fact, with a single exception, I 
cannot be sure that it has ever been mentioned by any previous writer on this sub- 
ject. In his paper on the origin of the test cells, Morgan (1890) figures and de- 
scribes this “peripheral zone of protoplasm” in an unidentified species of C/ave//zua 
