54 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
the anterior cells. The products of this division are nearly equal in size but are 
qualitatively dissimilar, the dorsal one (B®) containing less of the yellow proto- 
plasm and more yolk than the ventral one (B**). This difference between these 
daughter cells is plainly visible in the living condition, the dorsal cell being a 
fainter yellow than the ventral one (figs. 43-48, e¢ seg.); in preparations the dorsal 
cells always stain more deeply than the ventral ones, owing to the greater quantity 
of clear protoplasm which they contain (text fig. XXII). This difference in the 
constitution of these cells corresponds to a difference in their fate; the dorsal cells 
give rise to mesenchyme, while the ventral ones produce some of the muscle cells of 
the tail of the tadpole. 
The division of these twelve cells of the dorsal hemisphere are practically Ea: 
chronous, and they advance the egg from the 32-cell to the 44-cell stage. А little 
later the second cell of the crescent on each side of the mid-line (B**) divides, its 
spindle standing in a nearly dorso-ventral direction (figs. 45, 127-129). Тһе dorsal 
daughter cell (B77) in this case also contains less yellow protoplasm and more yolk 
than the ventral one (В?*), and like the cell which immediately adjoins it anteriorly 
(B'?) gives rise to mesenchyme, while the ventral moiety becomes a muscle cell. 
By this division the mesenchyme and muscle substance of the crescent are finally 
and completely segregated into separate cells, and the number of cells in the cres- 
cent is advanced to ten, and in the entire egg to forty-six. This division of the cell 
В“ is sometimes delayed until the cells of the ventral hemisphere are dividing 
(figs. 127—129), and а 46-cell stage is therefore not always present. Тһе division ` 
of the last remaining cells of the dorsal hemisphere, the middle cells of the crescent 
(B*? Б), is delayed until divisions are well advanced in the ventral hemisphere, 
and it may occur even after the ventral cells have divided (fig. 47). I do not find, 
therefore, that there is commonly a 48-cell stage such as Castle describes. 
The divisions of the cells of the ventral hemisphere are all synchronous, as 
figures 124 to 129 show. Тһе direction of the spindles in the different cells is so 
different that it is difficult to give an exact description of them. In the four median 
cells which surround the animal pole (а®®, b** and their fellows of the right side) 
the spindles are transverse; the spindles are also nearly transverse in the most 
anterior and most posterior pairs of cells (a^, 97); in the only other pair of cells 
which meet along the mid-line, the second pair in front of the animal pole (а®5), the 
spindles are nearly antero-posterior. In the other three pairs of cells of this hemi- 
sphere (a^*, 1%, b®*) the spindles are oblique in position, and their directions сап be 
best шине by consulting the figures (z. figs. 124, 196). Тһе most anterior 
pair of cells (a^?) are neural plate cells; these cells divide transversely (figs. 124— 
126), forming a transverse band of fous cells just above the equator; on each side 
of these a single cell (а79, fig. 150) is added at the close of this cleavage-—-hich 
completes the number ot. neural plate cells that are derived from the ventral 
hemisphere. Іп figure 130 the band of six cell (a??, a", а? and their fellows of А 
the right side) which lie around the anterior border of the ventral hemisphere are 
these neural plate cells. 
All the divisions of the cells of the ventral hemisphere a are equal, and all 
