ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 69 
In this process the ectoderm does not, for some time, close up the notch at the pos- 
terior end of the blastopore (fig. 52); this is one of the last steps in the process 
of closure. 
The overgrowth of the anterior lip continues until it has covered about three- 
quarters of the dorsal face; meanwhile the animal pole is shifted nearer to the 
point of greatest curvature at the anterior end, and the blastopore is transported 
from the dorsal side toward the posterior end. In this process the rows of muscle 
cells which at an earlier stage stretched from the posterior pole to the second 
cleavage plane, and were antero-posterior in direction, are tilted up at their anterior 
ends and pushed backwards until they lie at the hinder end of the embryo and run 
in a dorso-ventral direction (figs. 52, 53, 56, 157). This complete change in the 
direction of the rows of muscle cells I found most perplexing and difficult to under- 
stand. In early stages the crescent, and the mesoderm cells which form from it, lies 
just below the equator of the egg, and in the antero-posterior plane; in these later 
т stages the muscle cells are transverse to the anterior-posterior axis. А detailed 
ЕЕ study of intermediate stages shows how this change is brought about. After the 
: closure of the anterior part of the blastopore, corresponding to the bar of the T, 
the anterior lip does not overgrow the blastopore groove (stem of the T) and its 
lateral walls, which are composed of the muscle cells; on the other hand, these 
lateral walls lie at a higher level than the anterior lip, and the continued growth 
of this lip pushes the muscle cells and the groove before it. Аз the posterior lip 
remains stationary during this process it happens that the entire dorsal portion of 
the posterior quadrants is tilted up in front and pushed backward until it forms the 
posterior end of the embryo, the posterior lip becoming vental and the anterior lip 
dorsal in position. Thus the blastopore groove, which lay on the dorsal side pos- 
terior to the middle, comes to lie at the posterior end of the embryo and the walls 
of the groove, containing the muscle cells, come to be terminal in position and ver- 
tieal in direction (figs. 50—53). The mesenchyme cells, as well as the caudal 
endoderm, lie at so low a level that they are not disturbed by the overgrowth of the 
anterior lip, consequently the rows of these cells still preserve an antero-posterior 
direction (fig. 157). Thus the mesenchyme and muscle cells, which in earlier 
stages lay side by side, come to be separated anteriorly, and only remain in соп- 
tact with one another at the hinder end of the strand of caudal endoderm cells; 
the mesenchyme cells in this region are derived from the median part of the cres- 
cent and they ultimately become separated from the remaining portion of the 
mesenchyme which comes to lie in the trunk (figs. 161—167). | 
These general ehanges in the shape of the embryo at this stage are accom- 
panied by divisions of many of the cells, some of which we may now consider. In 
all the ectoderm cells the ninth cleavage is nearly synchronous; in the posterior 
quadrants the spindles are approximately antero-posterior in direction, and the 
same is true for the two hindmost rows of the anterior quadrants, but in most 
of the other cells of the anterior quadrants the spindles are transverse, thus it 
happens that the animal pole is shifted forward (fig. 149). Ав compared with 
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