118 



LEECHES 



have little yolk, gastrulation may be possible by invagination, the 

 macromeres forming the endoderm and the micromeres the ecto- 

 derm and mesoderm but if the macromeres are large and yolky 

 the micromeres multiply and spread down around the outside of 

 the macromeres bringing about gastrulation by epiboly (Fig. 69). 



Ectoderm 



Mesoderm 



Anus 



Fig. 69. Gastrulation and formation of the trochophore larva 

 in polychaetes. (a) section through a late cleavage stage ; (b) the 

 micromeres spread over the macromeres ; (c) early trochophore 

 with mesoderm stem-cells budding off a row of mesoderm cells 

 between the ectoderm and endoderm; (d) elongating trocho- 

 phore with mesoderm cells taking up a vertical position. From 



Waddington, 1956. 



The fate of each individual blastomere can be predicted accur- 

 ately and isolated blastomeres tend to develop only into those 

 organs which they would have formed if left undisturbed. It has 

 therefore become usual to regard the egg as a mosaic of localized, 

 organ-forming areas but it is a concept which must not be pushed 

 too far, for many spirally cleaving eggs possess a limited ability 

 to regulate the fate of the blastomeres according to circumstances. 



In polychaetes the blastomere D is particularly important for 

 most of the ectoderm and mesoderm of the adult worm are derived 

 from its descendants 2d and 4d. 2d is known as the somatoblast ; 



