CLEAVAGE AND DIFFERENTIATION 



II 



Fig. 55 

 Development of isolated blastomeres of Patella, a, An isolated i/8 blastomere 

 (micromere of the first quartet: i). b, c, Its division into two cells: i^ and i-. 



i" and i^-, 1 2^ and i--; one of these (i^^) 



d. Their division into four cells 



is an apical rosette-cell, another (i^-) is a " Molluscan cross" cell, and the 

 remaining tw^o (i^^ and i^^) are trocho blasts (stippled), e, Resulting larva 

 24 hours old, containing an apical organ (formed from derivatives of i^^), inter- 

 mediate cells (derivatives of i^-), and four cilia-bearing trochoblasts, the results 

 of division of i^^ and i". These cells are exactly those to which 1/8 micromere 

 would give rise in normal development in a whole embryo. /, An isolated 1/16 

 blastomere (primary trochoblast, i^). g, h, Its division into two cells (i-^ and i--). 

 /, Their division into four cells (i-^^ and i-^- ; i^-^ and i"^).j, The same, 24 hours 

 later ; each of the four cells (trochoblasts) has put out cilia, but divides no further. 

 These cells are exactly those to which a 1/16 blastomere (primary trochoblast) 

 would give rise in normal development in a whole embryo, k, A pair of trocho- 

 blast cells, the only product of an isolated 1/32 blastomere (trochoblast: i-^ or 

 T--). /, ?n, Isolated 1/64 blastomere (trochoblast: i-^^ or i-^-, i--\ i-"); such a cell 

 puts out cilia but does not divide. Ji, A pair of " secondary trochoblast cells," the 

 product of an isolated 1/32 blastomere (i^-, " Molluscan cross " cell). o,p, Isolated 

 1/64 blastomere ("secondary trochoblast," i^'"); such a cell puts out cilia but 

 divides no further. (From Jenkinson, Experimental Embryology , Oxford 1909, 

 after Wilson.) 



8-2 



