184 TORRE Y 



The rapidity with which the divisions have taken place in 

 these cells illustrates Lillie's ('95) principle that the rate of the 

 cleavages in the protoblast of an organ is directly correlated 

 with the time that the organ becomes functional. The cell 

 3<r?'o, after two divisions, also gives rise to mesenchyme. The 

 first division has already been described. In the second the 

 spindle first lies meridionally in both the a and b quadrants (PI. 

 II, Fig. 18), as in Podarkc but later rotates to a dexiotropic po- 

 sition. The ectomesoblast cell, 3% o. 2' divides equally as it sinks 

 in (PL II, Figs. 22 and 23). For a description of the later his- 

 tory of all these cells the reader is referred to the section deal- 

 ing with the mesoblast. The corresponding cell in the b quad- 

 rant plays a prominent role in the formation of the oesophagus. 

 The upper sinistral cells (3<^i, etc.), have all been seen to divide 

 dexiotropically (PI. II, Fig. 18). 



d. ^th and ^tli Qjtartets. — The posterior member of the 

 fourth quartet {^d^ is first to divide (PI. II, Figs. 14 and 19). 

 This is the first true bilateral cleavage in the ^^^ and does not 

 take place until the embryo consists of over a hundred cells. 

 The right product of this division is slightly larger than the left. 

 Soon after their formation the nuclei of the two cells (M^ M) 

 become shoved up near the surface by the pressure of the in- 

 vaginating entomeres (PI. II, Fig. 20). From them are ulti- 

 mately formed the right and left coelomesoblast bands. They 

 sink in immediately after the entoblast-plate and bud off two 

 small cells toward the blastopore (PI. II, Figs. 23 and 24; 

 Text-Fig. S, E). The other members of the quartet have al- 

 most invaginated (PI. II, Fig. 18), but return to the surface to 

 divide (PL II, Fig. 21). Child ('01) explains such a return by 

 the turgor of the cells and their adhesion to the connecting cells 

 of the surface layer. Just before sinking in, that part of the cell 

 at the surface becomes swollen and is connected with a nucle- 

 ated swollen part within the cleavage cavity by a narrow bridge 

 (PL II, Fig. 22). The cells are thus dumb-bell-shaped. They 

 later form a part of the entoblast. 



The fifth quartet is formed when the embryo consists of about 

 S6 cells (PL II, Figs. 17 and 18). The cells are somewhat 



