CELL-LINEAGE. 



9 



be followed of the progressive inwandering of the pigment-cells 

 {Fig. 4) to form the narrower posterior part of the pear-shaped 

 archenteron, while the anterior part is developed from the four 

 macromeres (entomeres) as is proved by the fact, among oth- 

 ers, that the fat-drops are found lying in its wall. There is no 

 possibility of mistaking the fact that the pigment-cells actually 

 form the archenteric wall, for their outlines can easily be seen 



Fig. 4, Nereis. Sagittal sections of larvce. A, trochophore (60 hours), showing 

 inwandering of the pigment-cells at the lower pole; stomodieum and neural plate 

 at the right; jB, larva of 4^ days, showing the pigment-cells at/. 



and the pigment-granules are found throughout the whole thick- 

 ness of the wall (Fig. 4, B). The pigment-cells are, therefore, 

 not mesoblastic, but are ciitoblast-cclls. 



In so far as the pigment-cells are derived from the macro- 

 meres (entomeres), this is exactly what we should expect. 

 That cells derived from the "primary mesoblasts " should 

 enter into the formation of the archenteron is howev^er a sur- 

 prising result ; and it is, therefore, highly important to make 



