SPIRALLY CLEAVING EGGS 



95 



a pocket-shaped primitive gut, v^hich gradually elongates until the whole 

 embryo assumes a shape like an old-fashioned peg top. There is usually an 

 apical tuft of cilia, and a strong ciliated band (the prototroch) more or 

 less at the equator. Meanwhile the 4^ mesoderm cell has been covered 

 over by ectoderm, lying in the corner at the boundary between ectoderm 

 and endoderm. It divides into two, and each of these daughter-cells gives 

 off a series of endomesoderm cells. In molluscs, these again become 

 scattered, but in annelids they remain joined together as two long bands. 



Ectoderm 



Ciliated 



"Prolotroc. 



Blastopore 



Mesoderm 

 stem-cells 



Mesoderm 



Figure 6.1 

 Gastrulation and formation of the trochophore larva in spirally cleaving 

 eggs. A shows a section through a late cleavage stage, the micromeres 

 lying directly on top of the larger macromeres, the slight space between 

 them corresponding to the cavity of the blastula. Some cells from 2d and 4 J 

 (shaded) have sunk beneath the surface. B, the micromeres spread over the 

 macromeres, which are being drawn up into the embryo, forming the 

 beginning of the primitive gut. C is an early trochophore ; note the two 

 large cells, derived from 4^, each of which is budding off a row of meso- 

 derm cells which extends round the blastopore between the ectoderm and 

 endoderm. D, the trochophore is beginning to elongate, and the mesoderm 

 bands are swung into a vertical position (this is typical of certain worm 

 embryos, such as those o( Polychaetcs). A mouth has appeared, either as a 

 new opening where the primitive gut has broken through the ectoderm, or 

 in some forms by the blastopore becoming constricted into two. 



