POLYCHAETA 281 



the sexes is facilitated by the exchange of light signals, and in the 

 nereids the discharge of sperm may only be brought about by the 

 influence of a secretion from the swarming female. Discharge of 

 the gametes is nearly always followed by the death of the sexual 

 individual. 



The fertilized egg gives rise to an unsegmented larva, the trocho- 

 sphere, which is described in the next section. 



Development of the Polychaeta 



The cleavage of the egg in the Polychaeta and the Archiannelida, 

 the polyclad Turbellaria, the Nemertea and the Mollusca follows 

 almost exactly the same plan. Division occurs rhythmically, affecting 

 the whole or greater part of the blastomeres at the same time. The 

 first two divisions are equal, producing four cells (Fig. 196, 2) lying 

 in the same plane, which are called A, B,CyD; each cell in its further 

 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 oflf successively from the macromeres as A, B, 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 and the next to the right again. The cleavage is 

 therefore said to be of 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 com- 

 posed of cells differing notably in size and density from those of the 

 first three. Of the fourth quartette d^ (Fig. 196, 4) alone produces 

 the mesoderm, while the other three, «*, Z>* and c*, 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. 



