88 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
that with a few possible exceptions, which are by no means well established, the 
polar bodies are formed at the animal pole of the egg in all cases. This is a fact of 
the most general occurrence and of the highest significance ; it indicates that before 
or during the maturation of the egg there occurs a polar differentiation or localiza- 
tion of the egg substance of such a kind that in all cases the future ectoderm is 
formed at the maturation pole and the endoderm at the opposite pole. 
The apparent exceptions to this rule are few in number and may be examined 
in some detail; they are limited to the eggs of certain insects, Petromyzon, 
copepods, Ascarzs, echinoderms and ascidians. The only reason for supposing, as 
Korschelt and Heider (1903, pp. 545, 546) do, that the polar bodies are not formed 
at the animal pole in insects and in Petromyzon is that they here lie to one side of 
the pointed end of the egg; there is no proof that they do not lie at the middle 
of the ectodermal area. Hicker (1899) says that in the larger species of Cyclops 
* neither the place of formation of the polar bodies, the place of entrance of the 
sperm nor the position of the first cleavage spindle are preformed in the egg, but 
are secondarily determined by the position of the egg in the egg sack" (pp. 
193, 194). However, this egg is one which is not easy to orient, and it has by no 
means been proven that the plas bodies do not form in this case at the middle of 
the ectodermal area. Even if the justice of all of Hicker’s statements be admitted 
it has not been shown that the cleavage spindle may not rotate so as to cause the 
first and second cleavage furrows to pass through the maturation pole, as is usually 
the case. Such a rotation of the first cleavage spindle takes place in nematodes, 
and a somewhat similar rotation of the entire egg, after the formation of the first 
cleavage spindle, has been described by Bigelow (1902) in the case of Lepas, where 
it had previously been held that the first cleavage was equatorial. Hacker’s obser- 
vations do not show that the chief axis of the egg is not predetermined, and they 
certainly dg not prove that the maturation pole and the ectodermal pole do not 
coincide. 
In Ascaris megalocephala, Boveri (1887) observed that the second polar body 
is usually formed at some distance from the first “whether through wandering іп 
the protoplasm or through a turning of the entire egg I could not determine" (p. 
32). His figures (1888, pl. ТУ) show that the first cleavage furrow frequently 
passes through the point of attachment of the second polar body. The study of 
the cell lineage of Ascaris has shown that most of the ectoderm is segregated in 
one of the first two cleavage cells (the “ primary ectoderm cell” of Zur Strassen, 
1896). This would seem to indicate that in this animal the polar bodies do not lie 
at the middle of the ectodermal pole; however the relations of the maturation | 
pole to the ectodermal pole and to the first cleavage are not clear in this case, and 
it may not be impossible that Ascarzs may yet be found to conform to the general 
rule. 
As for the echinoderms, Wilson (1895) supposed бай indirect evidence that 
the maturation pole and the future animal pole did not usually coincide in 
Toxopneustes, and further that the chief axis of the egg was established only after 
