106 ORGANIZATION AND CELL-LINEAGE ОҒ ASCIDIAN EGG. 
they give rise. He calls particular attention to the fundamental resemblances 
between the eges of Dentalium and of Myzostoma in the matter of the polar 
lobe and the “pillar of protoplasm.” Furthermore this lobe is comparable to 
the polar rings of leeches and oligochztes. Such a lobe, although present in some 
annelids and mollusks, is not present in all of them, and this would at first thought 
seem to mark some important difference in localization. But the presence or 
absence of such a lobe probably indicates no fundamental dissimilarity in the locali- 
zation, but rather variations in the surface tension and fluidity of different eggs. 
Although there are many interesting differences between various annelids and 
mollusks in the size of the polar lobe, of the blastomeres and of larval organs, these 
differences mark variations in the proportions of parts rather than in the type of 
localization. In all known cases among annelids and mollusks corresponding organs 
arise from corresponding regions of the egg. 
It may be concluded also from the work of Wilson (1903) and Yatsu (1904) on 
Cerebratulus that the character of the localization in the nemertine egg is essentially 
like that of the annelid and mollusk, though many of the details of localization 
are less accurately known in this case than in the others named. 
2. Ctenophore Type. 
If Fischel (1903) is right regarding the localization which he ascribes to 
the unsegmented ctenophore egg there is one fundamental difference between the 
ctenophore and other animals whose types of localization are known. On the 
authority of Metschnikoff he derives the mesoderm (somewhat doubtfully it must 
be said) from the micromeres at the upper pole of the egg, and consequently in his 
fig. 21 (p. 708) he localizes the mesodermal material at the upper pole of the unseg- 
mented egg. A zone below this, reaching to the equator or a little lower, repre- 
sents the ectodermal substance, and in it is located the material for the ciliated 
plates. At the lower pole and in the central part of the egg is the material sub- 
stratum of the endoderm. In all other well established cases the ectodermal sub- 
stances lie near the animal pole, while the mesodermal and endodermal substances 
lie near the vegetal pole. Inasmuch as an apical sense organ is formed at the 
animal pole in ctenophores in much the same way as in annelids, mollusks and 
nemerteans, it is difficult to resist the conclusion that the localization of the ecto- 
dermal, mesodermal and endodermal substances in the ctenophore egg will ulti- 
mately be found to be similar to that which prevails in other types. 
9. Echinoderm Туре. 
The form of localization in the echinoderm egg, as shown by Boveri's 
(1901) work on Strongylocentrotus, is in many respects similar to the annelid- 
mollusk-nemertean type. In this case, however, the mesoplasm is located at the 
lower pole of the egg and is sorrounded by an equatorial zone of endoplasm, whereas 
in annelids and mollusks, after the first two cleavages, the endoplasm lies at the 
lower pole, and the mesoplasm on the posterior side of this pole, and in one only of 
