ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 97 
clear protoplasm at each pole of the egg with a broad pigment band around the 
equatorial region. The clear polar areas, the lower of which forms a prominent 
lobe, Wilson regards as comparable with the “ polar rings" of leeches and oligo- 
chztes. In the course of development the upper white area is allotted to the three 
quartets of ectomeres; the middle pigmented zone is mainly allotted to the four 
basal entomeres, while the lower zone passes mainly into the first somatoblast (2d), 
and possibly also into the second somatoblast (4d) and the left posterior micromere 
(3d). This work is the most complete and important which has yet been done on 
the subject of cytoplasmic localization and it firmly establishes the fact that differ- 
ent substances and areas of the unsegmented egg are causally related to different 
organs and parts of the larva. 
It is doubtful whether any other case of cytoplasmic localization hitherto 
reported is more remarkable than that which has been described in the preceding 
pages for the ascidian egg. The most striking features of this localization are the 
great differences in the substances localized, the manner in which this localization 
is accomplished and its bilateral character. 
(1) The first of these features is the result of the different pigments which 
are associated with the different kinds of protoplasm, and which mark out as on a 
map the various germinal areas of the egg. In Cyn/hza the pigment in the periph- 
eral layer of protoplasm is yellow, the yolk is a blueish gray, while the protoplasm 
which escapes from the germinal vesicle is colorless. Not the pigment but the pro- 
toplasm with which it is associated is of differential value, for the pigment may 
differ most remarkably in different genera of ascidians, but the organs which arise 
from similar areas are in all cases similar. What has been said of the pigment may 
also be said of the yolk; this inert substance is not in itself of differential value, 
but it lies in a definite region of the egg and probably in a particular kind of proto- 
plasm, which it marks out as the yellow pigment does the peripheral layer. 
Of these three kinds of protoplasm the yellow (mesoplasm) goes almost entirely 
into the muscle and mesenchyme cells, though a small portion of it may be found 
around the nuclei of other cells, the clear protoplasm (ectoplasm) is chiefly distribu- 
ted to the ectoderm and the gray yolk-laden protoplasm (endoplasm) to the endo- 
derm, though here also some of these substances are distributed to all the cells. It 
is not to be supposed that these three kinds of protoplasm are the only ones present 
in the egg, rather it is probable that others are present which are not visibly distin- 
guishable. Іп fact, soon after the cleavage begins, it is noticeable that the proto- 
plasm in the dorsal part of the crescent is a fainter yellow than that in the ventral 
part, while from the time of the fertilization onward the middle of thé crescent is 
marked by a small area of clear protoplasm (о. p. 21); the deeply pigmented 
1 Several years ago I suggested 2 1897, р. 39) that the yolk lobe (“ polar lobe,” Wilson) 
em icem to the polar rings of leec 
should be observed that n names are given with reference to the part which these 
different mitis of the oóplasm play in the rbi cene of the animal; the peripheral layer Pei the 
ovoeyte, which would be called ectoplasm if the ovocyte alone were under consideration, is } mesoplasm 
when regarded from the standpoint of its fate i in development. 
13 JOURN. А. N. S. PHILA., VOL. XIII. 
