414 



ZOOLOGY 



SECT. X 



second set, somewhat larger than the others, and differing from 

 them in its subsequent history, is termed the first somatoblast 

 (som. 1); & second somatoblast (som. 2) is soon given off from 

 the same megamere that gave origin to 'the first. 



The germinal layers are now all established. The micromeres 

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

 all its derivatives, to the cerebral ganglion and nerve cord, to the 1 

 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. The micromeres extend.. 



micro 



.micro 



macro 



micro 



som.l 



micro 



macro 



som.2 



som.l 



Ft \. :5-2(i. Nereis. Early stages in the development. A, lateral view of eight-celled stage ; 

 Ji, the same from above ; (.', stage of the formation of the first somatoblasr ; />. stage at which 

 both somatoblasts are present; -macro, megameres; micro, micromeres; torn. l,gom. 2, first 

 and second somatoblasts. (After Westinghausen.) 



as they multiply by division, at first as a cap of small cells over 

 the upper pole of the embryo ; eventually the cap extends itself 

 so as completely to cover the four megameres and the descendants 

 of the somatoblasts except at one point, the blastoporc, at the 

 lower pole, where the investment remains for a time incomplete. 

 When the blastopore closes, the process of epib'6lic gastrulation is 

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

 p/(i.te, 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. 327, A, 





