ORGANIZATION AND CELL-LINEAGE OF ASCTDIAN EGG. 63 



direction of the spindles in these cells is indicated by the equatorial plates repre- 

 sented in figures L98 to 203. In the posterior quadrants all the spindles are 

 nearly antero-posterior in direction, except in those three ventral cells on each side 

 (b 7,9 , b rj ". W "i which lie nearest to the dorsal surface and between the muscle cells 

 behind and the neural plate cells in front (figs. 201, 203); in these cells the spin- 

 dles are nearly vertical. In the anterior quadrants the spindles are antero-pos- 

 terior in a transverse row of four cells which lies just in front of the animal pole 

 (a 7-16 , a'''i: in the row of four cell just in front of this the spindles are transverse 

 (a ;i \ a' "l: they are also transverse in a single pair of cells which meet at the mid- 

 line just in front of the last mentioned row (a'"); in the most anterior row of the 

 ventral hemisphere, consisting of six neural plate cells (a 7 - 10 , a 7,9 , a 7-13 ), the spindles 

 are dorso-ventral in position, therefore, at the close of this division there are 

 twelve neural plate cells in the ventral hemisphere, arranged in two rows of 

 six cells each. 



All of these divisions of the cells of the ventral hemisphere are synchronous, 

 equal and non-differential, and they increase the number of cells in the ventral 

 hemisphere to sixty-four and bring the whole number of cells in the embryo up to 

 one hundred and eight. 



Very soon after these divisions in the ventral hemisphere the posterior muscle 

 cell (B 7-8 ) and one of the mesenchyme cells (B 7,7 ) of each side divide; in the former 

 the spindles converge posteriorly and ventrally toward the median plane, in the 

 latter posteriorly and dorsally. By this division two mesenchyme and two muscle 

 cells are added to the total in the embryo which at this stage consists of one bun- 

 dred and twelve cells (fig. 133. 134 1 ) which may be tabulated as follows : 

 Ventral hemisphere 



Ectoderm . .8th generation, 52 cells. 



Neural plate 8th generation, 12 cells. 

 Dorsal hemisphere ' 



Endoderm 7th gen., 10 cells. 



Chorda .... 8th generation, 8 cells. 



Neural plate . 8th " 8 



Muscle .... 8th 8 " 



Mesenchyme 8th " 8 " 7th " 6 " 



8th generation, 96 cells. 7th gen., lb cells. 



112 cells. 

 The only cells in the entire embryo which have not passed into the eighth 

 generation at this stage are the ten endoderm cells, and six mesenchyme cells, two 

 of which are anterior, and four median and posterior: all of these cells except two, 

 the small posterior mesenchyme cells ( l? 7 !. divide soon after this stage and thus 

 pass into the eighth generation, but not until after other cell- have passed into 

 the ninth. This stage, therefore, max be taken as representing, as nearly as may 

 be found, the close of the seventh cleavage ami the transition to the eighth. 



'The division in a pair of mesenchyme cells B 7-7 ) of this stage i> not completed; therefore, in 

 the explanation of figures, tin- is called a LlO-cell stage. 



