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CASTLE: EMBRYOLOGY OF CIONA INTESTÍNALIS. 22 
The section seen in Figure 15 shows that cleavage has progressed more 
rapidly from the ventral than from the dorsal surface. This is to be 
explained by the richer supply of protoplasm on the ventral surface. 
A study by reconstruction or otherwise of a series of sections through 
an egg in this stage invariably shows that cleavage has also progressed 
with unequal rapidity from the two ends of the embryo. 
That end at which cleavage is more advanced is destined to become 
the posterior end. In this case also the inequality in rate of cleavage 
is attended (probably caused) by an inequality in the distribution of 
protoplasm. The protoplasmic cap of the ventral hemisphere is always 
thicker at the future posterior end of the embryo than at the anterior 
end, and as the first cleavage plane cuts the egg, this accumulation of 
protoplasm migrates in between the two blastomeres, its presence prob- 
ably being the accelerating force in the separation of the blastomeres. 
After the first cleavage is completed, the protoplasm, which had mi- 
grated in between the blastomeres, again returns to the surface and takes 
up a very definite position on the adjacent faces of the blastomeres just 
below the equator of the egg. (See Plate III. Fig. 17, x.) 
This region appears in the living egg as a clear area, and marks the 
spot where arise later the small flattened posterior cells found so useful 
in orientation by Van Beneden et Julin and others. That this clear 
area is the region of their formation I have been able to establish by 
continuous observations of the living egg, controlled and completely sup- 
ported by the study of preparations. The thickened spot in the protoplas- 
mic cap of the ventral hemisphere at the beginning of cleavage, which seems 
to determine the posterior end of the embryo, T believe to be caused by the 
entrance of the spermatozoón. It is evident that the spermatazoün, 
unless it enters exactly at the ventral pole of the vertical axis, must lie 
upon entrance nearer to one end of the egg than to the other, supposing 
that it is in the median plane and ventral hemisphere of the embryo. 
The nearer end, I believe, becomes the posterior end of the embryo, 
and is determined for that fate by the accumulation of protoplasm in 
the region of entrance of the spermatazoón. It is impossible to say in 
any particular case exactly where the spermatazoón has entered the egg, 
for its presence there cannot be detected until it has begun to form a 
yolk-free area in the egg. However, I have never observed a case in 
which the spermatozoón did not give evidence from its position of having 
entered the egg excentrically with reference to the lower pole of the ver- 
tical axis. Hence I conclude that cases of entrance a£ that pole, if they 
occur, are extremely rare, 
