Il8 MCMURRICH. [Vol. XI. 



counting from behind forward, the two cells on either side of 

 the median line remain as before, but in the three lateral rows 

 of either side a division of the original cells has occurred, there 

 being on one side six cells arranged in two parallel rows, while 

 on the other side the middle cell has not yet divided, so that 

 only five cells are seen. 



In the next row, or since the various transverse rows have 

 evidently a segmental significance, in the next segment {aV), 

 the multiplication of the mesoderm cells has progressed still 

 further, there being now four cells in a single row represent- 

 ing the original two cells on either side of the median line, 

 while the arrangement indicated for the lateral cells in the 

 segment next behind is complete, and two parallel rows consist- 

 ing of three cells each are to be found. In the next segment 

 anteriorly (///") there are still four median cells, but the lateral 

 rows are again in division, the division apparently affecting the 

 anterior of the two rows already present and leading to the 

 formation of three parallel rows each consisting of three cells. 

 In this segment there is to be seen the first indications of the 

 budding out of the limbs, these structures becoming more pro- 

 nounced as one passes forward, and at the same time the 

 multiplication of the mesoderm cells progresses rapidly, so that 

 in the jDreparation at present being described it is impossible to 

 trace any regularity in the division of these cells. In the most 

 anterior of the segments figured, however {tJ?), as well as in the 

 two which succeed it five masses of mesoderm cells are to be 

 distinguished: (i) a mass occupying the median line of the 

 segment and unquestionably derived from the two mesoblasts 

 situated on either side of the median line, (2) a mass on either 

 side which corresponds to the limb bud, below which it l.ies, 

 and (3) a mass, also paired, which lies just lateral to each limb 

 bud. 



The regularity of the division of the mesoderm cells is shown 

 more satisfactorily in Fig. 63, which represents the posterior 

 extremity of a very slightly younger embryo of CymotJioa. Pos- 

 teriorly the cells were not perfectly distinct, some of them 

 apparently having remained adherent to the yolk when the 

 embryo was removed from it. In the fourth segment, counting 



