52 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
Divisions begin in the ventral hemisphere before they are finished in the dor- 
sal (figs. 118, 191). In the most anterior and posterior pairs of cells of this hemi- 
sphere (a??, 194) the spindles are nearly parallel with the median plane; in the two 
remaining pairs of cells (a5* and b>’) the spindles are oblique from posterior-ventral 
to anterior-dorsal (figs. 118, 191). The division of the, anterior pair of cells (а, 
figs. 118, 119) gives rise to a couple of cells (аё, a^?) which lie just above the equa- 
tor and in contact with the chorda-neural-plate cells of the dorsal hemisphere. 
Later development shows that these cells form part of the anterior portion of the 
neural plate; the only other cells of the ventral hemisphere which enter into the 
formation of pos plate are portions of the cells а, which lie on the lateral borders 
of the cells aë. АП of the cells of the ventral hemisphere are of practically the 
same size and анна each consists of a superficial layer of protoplasm, in 
which the nucleus les, and a deeper layer of yolk, the cells of this pole being 
decidedly protoplasmic as compared with those of the opposite pole. 
The result of this cleavage is the formation of sixteen cells in each hemisphere 
which may be tabulated as follows : 
Ventral hemisphere 
14 ectoderm cells, protoplasmic. 
2 neural plate cells, protoplasmic. 
Dorsal hemisphere 
6 endoderm cells, yolk laden. 
4 chorda-nerve * cells, yolk and protoplasm. 
6 mesoderm cells, yellow protoplasm or crescent substance. 
32 cells. 
At the close of this cleavage the cells of the чш epos are smaller in 
superficial area than those of the dorsal hemisphere ; when viewed from the ventral 
pole the dorsal cells are seen around the entire кайны of the egg, except at a 
point on the right and left sides where a single ventral cell (b**, b®*) occupies the 
periphery; this is the only cell of the ventral hemisphere which сап be seen from 
the dorsal pole (figs. 119, 192, 193). А similar condition prevailed at the close of 
the preceding cleaving (figs. 116, 117, 190), the only cells of the ventral hemi- 
sphere which could be seen from the dorsal pole being b*? and bë. This condition 
may be traced still farther back to the 8-cell stage (figs. 108, 110, 184) where the 
ventral cells are smaller than the dorsal ones and where the only portion of the 
ventral hemisphere which lies below the general plane of the equator is that part 
of each of the posterior-ventral cells (b*?, bt?) which meets the anterior-dorsal cell 
in the cross furrow (figs. 108, 184). 
At the close of the fifth cleavage the superficial area of Tüs ventral cells is 
smaller than at any preceding stage kad that of the dorsal cells is greater; this is 
due to a change in the shape of the cells, the ventral cells becoming long and E; 
! Throughout this paper the cells which are to give rise to chorda, nerve, muscle and mesen- 
chyme are, for the sake of brevity, frequently referred to as if they had already given rise to these 
structures. 
