ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 105 
there is remarkable uniformity in the localization of the substances of the germinal 
layers in all of these groups, the ectodermal substances being located in the upper 
hemisphere, and the endodermal and mesodermal in the lower hemisphere of the 
egg. But in the localization of important organ bases there are many differences 
between these phyla. 
1. Annelid-Mollusk Type. 
The pattern of localization in annelids and mollusks is very similar during the 
cleavage stages and, so far as can be judged from present knowledge, it is much the same 
in the unsegmented eggs of these two phyla. The fact that the ectoderm, mesoderm 
and endoderm come from cells which are identical in origin, position and number; 
that the umbrella, prototroch, cerebral ganglion, sub-cesophageal ganglion, mesoder- 
mal bands, blastopore, stomodzeum and intestine come from corresponding region of 
the egg in the two groups, these facts speak strongly in favor of the regional homolo- 
gies of the eggs of these phyla, whatever may be thought of their cell homologies 
(Conklin, 1897; Child, 1900). But regional homologies as well as cell homologies 
must be based upon similarities of germinal localization, and we would, therefore, 
be justified in concluding that the types of localization were similar in the unseg- 
mented eggs of annelids and mollusks even in the absence of any direct knowledge 
upon that subject. But the experiments of Crampton (1896) on ///yanassa and 
of Wilson (1904) on Dentalium as well as the observations of Lillie (1899, 1901) 
on Unio, and my own observations on localization in the eggs of Crepidula, Physa, 
Planorbis and Limnea furnish considerable information as to the time, the manner 
and the nature of localization in the molluscan egg during and before cleavage, while 
numerous works on the cell-lineage of the annelids as well as the observations of 
Wheeler (1897), Driesch (1896) and Carazzi (1904) on the unsegmented egg of 
Myzostoma show that the nature of localization is here very similar to that found 
in the mollusks. 
In all of these cases the only formative substances which are directly recogniz- 
able before cleavage are those of the future germ layers. In the main the ectoder- 
mal substances are located in the upper hemisphere and the endodermal in the 
lower, though Wilson (1904) has found that the apical organ does not form in the 
larva of Deztalzum when the polar lobe at the vegetal pole is removed. Тһе meso- 
dermal substances are also located іп the lower hemisphere, and since the primary 
mesoderm cell (4d) always come from the left posterior macromere of the 4-cell 
stage and from the posterior blastomere of the 2-cell stage it may be inferred that 
immediately before cleavage it lies posterior to the vegetal pole; whether it may be 
located exactly at the vegetal pole in still earlier stages and then later shift to the 
posterior side, as in ascidians, cannot be determined at present. 
When a polar lobe is present the mesodermal substance is probably located in 
it. Crampton (1896) found in ///panassa that the mesoblast cell (4d) did not form 
when the lobe had been removed; Wilson (1904) holds that in Dez/a/zum the sub- 
stance of the lobe is allotted to both the first and second somatoblasts (2d and 4d), 
and that its size is proportional to the size of those cells and of the parts to which 
14 JOURN. А. N. S. PHILA., VOL. ХШ. С. 
