9'2 T. FUJITA. 



those of the first generation. The generation just mentioned then 

 divides, thus giving to a new which is the eighth. The ectoderm that 

 arise from these micromeres, may now be considered as well established. 

 At about the same time as the formation of the eighth generation of 

 micromeres, the TJr mesoderm divides into two lateral segments, at 

 which the bilateral symmetry of the body becomes quite apparent 

 (fig. 7, 8). Soon after this, the entodermic macromeres A, B and C, 

 undergo division and then they form the entoderm cells, together with 

 the .one that had arisen by division of the entomesoderm. The 

 mesoderm cells are afterwards successively budded off from the two 

 Urmesodenn cells alreadly mentioned and form two lateral bands as 

 is well known (fig. 9, 10, 11). 



The main points to be adduced from the above are as follows : 

 1st. The entomesoderm cell first establishes itself at the l9-cell stage. 



(4 generations of mic. + 3 mac). 

 2nd. The entomesoderm cell differenfciate into an entoderm cell and 



an TJfmesodenti at the 27-cell stage (G generations of mic. + 



3 mac). 

 3rd. Bilateral symmetry is fully established at the 33-coll stage (7 



generations of mic. + 3 mac.+ 1 urm.-\- 1 ent.). 

 4th. The TJnnesoderm cell segments into two lateral ones at the 



38-cell stage (8 generation of mic. + 2 X + 3 mac. + 1 ent.)- 

 5th. Entoderm cells formed by division of three macromeres are added 



to the one already present that was segmented off from the ento- 

 mesoderm cell. 

 6th. The mesoderm begins to be formed from the two lateral TJrmcso- 



dcrm cells at about 43-cell stage (8 generation of mic. + 2 X + 7 



ent. + 2 urm.). 



Explanation of Figures. 



ect. ectoderm. p. pola body. 



ent. entoderm. ur. urmesoderm. 



m. mesoderm. X cells arising from micromeres 



of the fourth generation. 



