PHYLUM ANNULATA 



521 



megameres, posterior in position, divides into two cells (B): 

 the so-called neuroncphroblast and mesoblast, the latter of which at 

 once divides into two. As shewn by their subsequent history, the 

 neuronephroblast and the mesoblast correspond respectively to the 

 first and second somatoblasts of Nereis. The former divides and 

 sub-divides to form two symmetrical groups of four cells each, 

 situated at the posterior pole. The number of micromeres 

 increases, at first apparently by division of the megameres. The 

 latter subsequently give off a number of small endoderm cells. 



The embryo now consists of the three large megameres with a 

 number of endoderm cells, a cap of small micromeres forming an 

 ectodermal layer which is extending over the surface, with, at the 

 posterior pole, two symmetrical groups of neuronephroblast cells 

 (four in each), and, somewhat deeper, the two mesoblast cells. 



mtfi 



FIG. 417. Six stages in the development of Clepsine. g. b. germinal bands ; r,ig. megameres 

 mi. micromeres ; mth. mouth. (After Whitman.) 



From each of the ten cells last mentioned new cells are given off 

 in front in such a way as to form ten rows of cells, five on 

 each side, four being derived from neuronephroblasts and one from 

 the mesoblast cell, These two sets of rows of cells constitute the 

 so-called germinal bands. From their subsequent fate it is clear 

 that they correspond to the mesoderm bands of Nereis phis the 

 neural plate. They grow forwards, the ectoderm extending with 

 them, over the endoderm and megameres. At first they diverge 

 widely, but their anterior ends subsequently meet towards the 

 anterior end of the embryo. Later the intermediate parts of the 

 bands, originally widely separated from one another owing to their 

 divergence during growth, approach one another and meet along 

 the middle line of the ventral surface. The germinal bands give 

 rise to the nerve-cord, the mesodermal segments, and the nephridia. 

 The layer of micromeres not only gives rise to the whole ectoderm 



