x PHYLUM ANNULATA 441 



constitute the ectoderm, destined to give rise to the epidermis and 

 all its derivatives, to the cerebral ganglion and nerve-cord, to the 

 oesophagus and rectum. The megameres eventually give origin 

 to the cells of the endoderm, forming the internal epithelium of 

 the alimentary canal. The second somatoblast gives rise 

 to the entire mesoderm of the Annelid and contributes a few small 

 cells to the endoderm of the intestine. As the micromeres multiply 

 by division, they form at first a cap 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 the middle 

 of the head-end projects a tuft of cilia (Fig. 362, A, ap. til.). 

 Encircling the body of the larva behind this is a thickened 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 

 stomodo&um (si) or rudiment of the mouth and oesophagus. The 

 anus (an) does not appear until later ; the position which it will 

 subsequent!}^ occupy is indicated at this stage by a pigmented 

 area (pig. ar) marking the point at which the blastopore becomes 

 closed. The first and second somatoblasts divide to form a mass of 

 small cells which extend on the ventral surface behind the prototroch 

 and mouth, constituting what is termed the ventral plate ; of this 

 plate the more superficial cells are descendants of the first somato- 

 blast ; and those situated more deeply are derived from the 

 second somatoblast. A superficial thickening of the ectoderm 

 along the middle of the ventral plate is the rudiment of the ventral 

 nerve-cord (neur. pi) ; the deeper cells divide and extend to form a 

 pair of mesoderm-bands or muscle-plates, from which the muscles 

 of the body- wall are developed ; the muscular layers of the -wall of 

 the alimentary canal are derived from certain of the same set of 

 cells which migrate inwards from the lower end. 



A pair of micromeres separated from the rest at an early stage 

 are destined to form the larval excretory organs, the head-kidneys or 

 larval nephridia : at first situated at the upper end, they sink 

 below the surface and migrate downwards till they come to lie below 

 the prototroch ; each then elongates, and a number of vacuoles 

 which have become formed in the interior coalesce in such a way 

 as to form a long, narrow canal. The embryo has now reached 

 the completed trochophore stage. 



The endoderm cells become arranged so as to bound a canal- 



