54 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



the anterior cells. The products of this division are nearly equal in size hut are 

 qualitatively dissimilar, the dorsal one (B 7,3 ) containing less of the yellow proto- 

 plasm and more yolk than the ventral one (B 74 ). 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, et 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 syn- 

 chronous, and they advance the egg from the 32-cell to the 44-cell stage. A little 

 later the second cell of the crescent on each side of the mid-line (B 6 - 4 ) divides, its 

 spindle standing in a nearly dorso-ventral direction (figs. 45, 127-42'.>). The dorsal 

 daughter cell (B 77 ) in this case also contains less yellow protoplasm and more yolk 

 than the ventral one (B 78 ), and like the cell which immediately adjoins it anteriorly 

 (B 73 ) 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 

 B 64 is sometimes delayed until the cells of the ventral hemisphere are dividing 

 (figs. 127-129), and a 46-cell stage is therefore not always present. The division 

 of the last remaining cells of the dorsal hemisphere, the middle cells of the crescent 

 (B 63 B 63 ), 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 121) show. The 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 (a 6-8 , b' is 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 65 . b a7 ); in the only other pair of cells 

 which meet along the mid-line, the second pair in front of the animal pole (a 65 ), the 

 spindles are nearly antero-posterior. In the other three pairs of cells of this hemi- 

 sphere (a 6-7 , b 65 , b 66 ) the spindles are oblique in position, and their directions can he 

 best appreciated by consulting the figures {v. figs. 124, L96). The most anterior 

 pair of cells (a 6,5 ) are neural plate cells; these cells divide transversely (figs. 124- 

 126), forming a transverse band of four cells just above the equator; on each side 

 of these a single cell (a 7 - 13 , fig. L30) is added at the close of this cleavage which 

 completes the number of neural plate cells that are derived from the ventral 

 hemisphere. In figure 130 the hand of six cell (a 7 ' 9 , a 710 , a 713 and their fellows of 

 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 are equal, and all 



