ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 89 
fertilization. However, the evidence in favor of this is not conclusive as Wilson 
admits. On the other hand, Boveri (1901) has shown in the most convincing man- 
ner that in Strongylocentrotus the polarity of the egg may be traced back to the 
ovoeyte, and that this polarity determines the gastrular axis. lt is, therefore, 
possible that in all echinoderms the polarity of the egg is predetermined in the 
ovary, and not after the maturation and fertilization, and that in all cases the 
maturation and ectodermal poles coincide.’ 
The most remarkable and apparently well established of these exceptions to 
the rule that the polar bodies are formed at the animal pole is that of the ascidians 
studied by Castle (1894, 1896), where the polar bodies were said to be formed at 
the vegetal or endodermal pole of the egg. However, this conclusion rests upon 
erroneous orientation, as I have shown in the preceding pages; in ascidians as in 
other animals the polar bodies are formed at the ectodermal pole. There are, there- 
fore, no well established exceptions to this general law.? 
In many cases it is known that the polar differentiation of the egg may be 
recognized while the egg is still in the ovary. Reference has just been made to 
the condition in .S/rongy/locentrotus in which the pole of attachment to the ovarian 
wall becomes the maturation pole of the egg and the ectodermal pole of the larva. 
Boveri says that in all known cases the pole toward which the germinal vesicle is 
eccentric becomes the animal pole. In Uzzo, Lillie (1900) has demonstrated that it is 
the free pole of the egg which becomes the maturation and ectodermal pole, while the 
pole of attachment becomes the vegetal pole. In a number of gasteropods ( Zzmnea, 
Succinea, Polygyra, Limax, Рауза. Planorbis, Ancylus) Y have found that there 
is a marked polar differentiation of the egg in the ovary, the germinal vesicle being 
eccentric toward the free pole of the ovocyte. I have elsewhere (1905) shown 
reason for believing that in dextral snails the polar bodies are formed at the free 
pole and in sinistral snails at the attached pole of the ovoeyte. In his work on 
Cerebratulus, Wilson (1903), found that the polar bodies were formed at the free 
pole of the ovocyte, and again in his recent paper on Dentalium (1904), he finds 
the side of attachment in the ovary represents the lower or vegetal hemisphere. 
We find then that the chief axis of the egg is very generally present in the ovocyte, 
and that the free side usually gives rise to the maturation апа ectodermal pole, 
while the attached side becomes the vegetal pole; but in echinoderms and probably 
also in sinistral gasteropods these conditions are reversed, the side of attachment 
becoming the ectodermal pole. 
In the gasteropods named above, I found it possible to recognize this polarity 
of the ovoeyte at a very early stage; in general it coicides with the “organic 
axis" (Van Beneden), or the “cell axis" (Heidenhain) z. e., the axis passing through | 
the centrosome or sphere, and the center of the nucleus. This cell axis is a general 
! However, Garbowski (1904) affirms that in Asterias glacialis the polarity of the egg is not 
M DM. even in the 8-cell and 16-cell stages, and that the blastomeres are equipotential up to the | = а 
500-cell s 
ve us (1897, p. 41-46) has discussed in ап adinirsble manner the apparent exce айы. to 
this law of polar differentiation and concludes that these "M are by no means well establ 
12 JOURN. А. М. 5. PHILA,, VOL. XIII. 
