PHYLUM ANNULATA 



451 



the entire mesoderm of the Annelid. As the niicromeres multiply 

 by division, they form at first a can of small cells over the upper 

 pole of the embryo ; eventually the cap extends so as completely 

 to cover the four megameres and the descendants of the somato- 

 blasts except at one point, the blastopore, at the lower pole, 

 where the investment remains for a time incomplete. When 

 the blastopore closes, the process of epibolic gastrulation is 

 completed. A thickening of the layer of ectoderm cells, the apical 

 plate, in the middle of what is destined to form the head-end of the 

 embryo, is the rudiment of the cerebral ganglion : in close relation 

 to it are formed a pair of pigment-spots, the larval eyes. From 



micro 



som.l 



micro 



macro 



som2 



soml 



Fio. 355.— Nereis. Early stages in the development. A, lateral view of eight-celled stage ; 

 B, the same from above ; C, stage of the formation of the first somatoblast ; D, stage at which 

 both somatoblasts are present; macro, megameres; micro, niicromeres; som. 1, som. 2. first' 

 and second somatoblasts. (After Westinghausen.) 



the middle of the head-end projects a tuft of cilia (Fig. 356, A, 

 ap. cii). Encircling the body of the larva behind this is a thick- 

 ened ridge, the prototroch (prot), the cells of which develop strong 

 cilia. Just behind the prototroch the cells of the ectoderm 

 become pushed inwards in the middle of what will eventually 

 become the ventral surface, so as to line a sort of depression or 

 pouch ; this is the stomodamm (st) or rudiment of the mouth and 

 oesophagus. The anus (<m)does not appear until later; the position 

 which it will subsequently occupy is indicated at this stage by a 

 pigmented area (pig. ar) marking the point at which the blasto- 

 pore becomes closed. The first and second somatoblasts divide 

 to form a mass of small cells which extend on the ventral surface 



G g 2 



