ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. Gl 



sal side, are directed obliquely forward, outward and ventralward (figs. 131, 132). 

 These divisions, when completed, give rise in the anterior quadrants to four chorda 

 cells (A 8 - 5 , A 8 - 6 , A 818 , A. & "), and to four neural plate cells (A 87 , A 8 - 8 , A. 815 , A 818 ) on 

 each side of the mid-line, which are arranged in two rows of eight cells each 

 running around the anterior border of the dorsal hemisphere (figs. 134, 200). All 

 these divisions are equal and non-differential. 



In the posterior quadrants the division of the anterior muscle cell on each side 

 gives rise to two daughter cells (IT'. I'> ss ). one in front of the other, which are alike 

 in size and quality. The mesenchyme cell (B 7r ) divides unequally and differentially 

 giving rise to a small yolk-laden cell (B 88 ), lying anterior, lateral and ventral to its 

 large sister cell (B 8 '). which is more protoplasmic (figs. 134. 200). The former is, 

 according to Castle, the "posterior chorda fundament" while the latter is mesen- 

 chyme. I have been unable to find sufficient evidence that this small cell is later 

 incorporated in the chorda, but on the other hand do not wish to deny that this is 

 the case. Since it is derived from the mesenchyme I prefer to class it with the 

 mesenchyme cells until its fate is more certainly known. These divisions advance 

 the embryo to the 7G-cell stage and the distribution and generations of the cells 

 may be summarized as follows : 

 Ventral hemisphere 



Ectoderm 7th generation 26 cells. 



Neural plate 7th " G " 



Dorsal hemisphere 



Endoderm 7th " 10 " 



Chorda . . . . 8th generation 8 cells. 



Neural plate 8th " 8 " 



. Muscle . . 8th " 4 " 7th " 2 



Mesenchyme 8th " 4 " 7th " 8 



a 



sth generation 24 cells. 7th generation 52 cells. 



- 



o 



76 cells. 

 The classification of the cells of this stage into chorda, neural plate, muscle 

 and mesenchyme. 1 must of course be based upon the later history of these cells, but 

 even at this early stage important differences may be recognized in the histological 

 characters of the cells named. In the living eggs of Cynthia the endoderm cells 

 are slate-gray in color and are filled with yolk; the chorda cells are lighter 

 gray and contain less yolk than the endoderm cells; the ectoderm and neural 

 plate cells are clear and protoplasmic ; while the muscle and mesenchyme cells 

 are yellow, the former being more deeply colored than the latter. In the main 

 my classification of the cells of this stage agrees with that of Castle; the most 

 important difference concerns the muscle cells which he classes as " neuro-muscular 

 cells." Owing to the striking color of these cells in Cynthia their later history can 

 be followed with relative ease; they ultimately give rise to the three rows of muscle 



See note p. 52. 



