248 THE INVERTEBRATA 



cleavage resembles the others and gives rise to one of the quadrants of 

 the embryo. D tends to be larger than the others and becomes the 

 dorsal surface of the embryo, while B is ventral, A and C lateral. The 

 next divisions (third, fourth and fifth) are unequal and at right angles 

 to the first two and result in three quartettes of micromeres being 

 divided off successively from the macromeres as A^ By C and D are 

 then termed. The region in which the micromeres lie is the upper or 

 animal pole of the embryo, while the macromeres form the vegetative 

 pole. The micromeres are not directly over the macromeres from 

 which they are formed but in one quartette they are all displaced to 

 the right, while in the next they will be displaced to the left of the 

 embryonic radius. The cleavage is therefore said to be oi radial spiral 

 type and successive cleavage planes are at right angles. At a later 

 period it is replaced by cleavage in which there is no alternation of the 

 kind described above, and the result is that the embryo becomes 

 bilaterally symmetrical. 



The rest of the description is drawn from the Polychaeta but can 

 be applied with slight modifications to the other groups. 



The cells of the first three quartettes give rise to the ectoderm of 

 the larva and of the adult. The sixth division, however, results in the 

 separation from the macromeres of a fourth quartette which is 

 composed of cells differing notably in size and density from those of 

 the first three. Of the fourth quartette d^ (Fig. 185, 4) alone produces 

 the mesoderm, while the other three, a^, b^ and t*, reinforce the 

 macromeres to form the endoderm. The mesoderm is, however, only 

 in course of differentiation during larval life and a larval mesoderm or 

 mesenchyme is produced from which particularly the musculature of 

 the trochosphere is fashioned. The mesenchyme is derived from the 

 inward projections of cells of the second and third quartettes. 



Gastrulation (Fig. 185, 7). The amount of yolk in the macromeres 

 determines the character of the cleavage within certain limits and the 

 type of gastrulation. In forms like Polygordius with very little yolk 

 the micromeres and macromeres are nearly the same size and gastru- 

 lation takes place by invagination ; in Arenicola, Nereis and nearly all 

 Polychaeta and all MoUusca the micromeres are much smaller than 

 the macromeres, and as they divide to form the ectoderm they grow 

 round the massive macromeres and an " epibolic " gastrula is formed. 

 The cells of the fourth (and fifth) quartettes approach each other 

 from the two sides. The mesoblast cell (^^) begins to withdraw from 

 the surface into the blastocoele, and the blastopore, that is the un- 

 covered surface of the macromeres , becomes much smaller and slit-like . 

 Eventually as gastrulation is completed the lips of the blastopore join 

 in the middle, the same cells meeting each other in every case, leaving 

 an anterior opening which becomes the mouth and a posterior, which 



