220 



INTERNAL FACTORS 



IV. i 



one end, and four cells (derived from a primary trochoblast) 

 provided with long cilia at the other. When the primary trocho- 

 blasts (1 a 2 1 d 2) are isolated each divides into four ciliated 

 cells, the cilia arranged as normally in a single row, but no further 

 (Fig. 1 29 a-e). If the daughter-cells of these trochoblasts (1 a 2. 1 

 1 d 2. 1 and 1 a 2. 21 d 2. 2) are separated they divide once 

 only (Fig. 129/), while their daughter-cells do not divide at all 

 (Fig. 129 g, //). The sister-cells to these (1 a 1 1 d 1) behave in 

 the same way, forming a closed larva with an apical organ at 



J 9 ll i 3 k 



FIG. 129. Patella. Development of isolated primary and secondary 

 trochoblasts. a, Primary trochoblast ; b, c, first division ; d, second 

 division ; e, after 24 hours ; /, pair of primary trochoblasts, the products 

 of division of either I' 21 or I 22 , after isolation ; g, h, single primary 

 trochoblasts, either I 212 , 1 CI1 , I 221 , or I 22 ' 2 ; i, a pair of secondary trocho- 

 blasts, the products of I 12 ; j, k, single secondary trochoblasts. (After 

 Wilson.) 



one end and secondary trochoblasts at the other, while the isolated 

 apical cells and secondary trochoblasts divide as often as they 

 would in the whole embryo and put out their cilia (Fig. 129 i-k, 

 Fig. 130 a-e). Finally, the whole first quartette becomes an ecto- 

 blastic larva with an apical organ at one end and a prototroch at the 

 other, but with no archenteron (Fig. ISO/) g\ The cells of the 

 second quartette (2 a 2 d) likewise form each a hollow ectoblastic 

 vesicle containing larval mesenchyme, ciliated secondary trocho- 

 blasts, and a few (preanal) ciliated cells (Fig. 131). 



