30 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
maturation are uniformly reported to take place at the animal, z.e., at the more 
richly protoplasmic pole, whereas in (ола they take place at the vegetative pole. 
. . . The statement made in the preceding paragraph presents a condition of affairs 
so directly contrary to that found in other groups of animals, as well as to what has 
been assumed by all previous writers to be the case in ascidians, that it requires the 
presentation of unmistakable evidence in its support. Such evidence I have to offer, 
both from the study of the living egg and from that of preparations.” 
What is this evidence? So far as it relates to the origin of the polar bodies at 
the vegetal pole it is twofold; (a) the polar bodies are formed at the yolk-rich 
pole, (b) this pole becomes the endodermal pole of the gastrula. As to the first of 
these propositions I have already shown that the germinal vesicle fades and the first 
maturation spindle appears at the protoplasmic pole (figs. 77, 78, 172). Only later, 
after the entrance of the spermatozoon, does the protoplasm flow away from this pole, 
leaving the maturation spindle closely surrounded ‘by yolk; still later, during the 
first cleavage, the protoplasm flows back again to near the center of the egg and at 
the close of this cleavage it moves still nearer to the pole at which the polar bodies 
lie (figs. 100, 102, 106, 107, 178); thereafter this pole is always the more richly 
protoplasmic. Therefore, except for a brief period after the fertilization and before 
the first cleavage, when the protoplasm is temporarily withdrawn from the matura- 
tion pole through the influence of the spermatozoon, the maturation or animal pole 
and the more richly protoplasmic pole are one and the same in ascidians as in other 
animals. 
As to the statement that the polar bodies are formed at a point which corresponds 
to the center of the dorsal or endodermal pole of the gastrula it is evident that 
unless the polar bodies have been actually followed through the development to a 
stage when the ectodermal and endodermal poles are unmistakable, this statement 
must rest upon indirect evidence furnished by a study of the cleavage stages. As 
a matter of fact, Castle has not figured nor described the polar bodies in any egg 
later than the 16 to 24-cell stage, whereas there is no trace of gastrulation in Czona 
before the 76-cell stage (fig. 200). Undoubtedly therefore Castle's evidence that the 
polar bodies are formed at the endodermal pole must be indirect rather than direct, 
and must be derived from the study and orientation of the cleavage stages. We 
may therefore turn at once to the evidences which led him to reverse Van Beneden 
and Julin’s orientation of these stages. So far as I am able to discover there аге, in 
addition to several minor considerations which could at best be considered only as 
confirmatory, two and only two general lines of evidence which he brings forward 
in favor of his contention. They are the following: 
(1) The hemisphere in which division is earliest as the egg passes from the 16- 
cell stage to the 32-cell stage, and from the latter to the 46-cell stage becomes later 
the ventral or ectodermal hemisphere of the embryo (1894, p. 206; 1896, pp. 229 and 
235). The second paper refers to this proposition as having been demonstrated in 
the first. What is this demonstration? So far as I can ascertain it consists merely 
in the assumption that the cells, which in the 16-cell and 52-cell stages divide earlier 
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