Жаң 
ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 59 
anterior and posterior quadrants is finally brought back to its right position and 
given its true designation. In all stages later than his figure 60 the designation 
А?" stands for the same cell in Castle’s figures and in my own. 
The changes in the designations of the cells which are brought about by this 
inversion of the orientation at the 48-cell stage may be most easily seen and appre- 
ciated by a reference to the accompanying text figures XXV and XXVI, where the 
designations of the cells, according to my interpretation, are given on the right side 
of figure XXV and on the left side of figure XX VI, while Castle's designations of the 
corresponding cells are given on the left side of figure XXV and on the right side 
of figure XXVI. Barring the exceptions mentioned in the preceding paragraph, 
Castle has followed with substantial accuracy the subsequent lineage of the dorsal 
hemisphere up to a stage of about one hundred and twelve cells, though always 
upon the basis of his erroneous lineage of the 48-cell stage. With the exception of 
a single pair of cells, I need not further explain my departure from Castle's nomen- 
clature of the later stages. This exception is the pair of small posterior mesen- 
chyme cells which Castle designates С, 07°; inasmuch as I find that they lie 
ventral to their sister cells, I shall designate them B*5, В? and their more dorsally 
placed sister cells BY, BY. 
With the completion of the sixth cleavage we reach a period when the gastru- 
lation is ready to begin. Already preparations for the gastrulation are apparent in 
the changing shapes of the cells of the dorsal and ventral hemispheres, in the rela- 
tive positions of the cells and in the directions of their divisions. ven the pecu- 
liar type of the chordate gastrula, with tts overgrowing anterior lip and 115 nearly 
stationary posterior one, 15 foreshadowed at a very early stage in the eccentric post- 
поп of the animal and vegetal poles in the two hemispheres of the egg. 
In the 52-cell and 64-cell stages it is apparent that the animal and vegetal poles 
do not mark the middle of the ventral and dorsal faces of the embryo. This was 
first noticeable in the 4-cell stage of Cynthia where the two posterior cells are 
smaller than the anterior ones. In the 8-cell stage the anterior-ventral cells are 
elongated antero-posteriorly, while the posterior-ventral ones are elongated trans- 
 versely ; this brings the animal pole still farther back of the middle of the ventral 
face. In the 16-cell stage there are two pairs of cells adjoining the mid-line in 
front of the animal pole and but one pair behind it; in the dorsal hemisphere there 
is one pair of such cells in front of the vegetal pole and two behind, but the most 
posterior pair is smaller than the others, so that the vegetal pole lies near the mid- 
dle of the dorsal face. In the 32-cell stage three pairs of cells adjoining the mid- 
line are in front of the animal pole, two behind it; while in the dorsal hemisphere 
there are two pairs of such cells in front of the vegetal pole and two behind it, 
though here again the most posterior cells are very small ones. Іп the 64-cell 
stage (fies. 139; 131) there are four pairs of cells adjoining the mid-line in front of 
the жаппа! pole and only two pairs behind it; while in the dorsal hemisphere there 
аге four pairs of such cells, both in front of and behind the vegetal pole, but the 
most posterior pair are the tiny mesenchyme cells (B™, B*5) which are partly cov- 
