THE PRIMARY SEGMENTATION. 



129 



metropolitan plane the lobster, the crab, the insect, in short 

 all the members of the great arthropod group, meet and 

 acknowledge their community of descent. Thus the great 

 branches of the genealogical tree represented in the larger 

 types first defined by Cuvier converge and meet in a common 

 trunk which bears the deep and enduring mark of metamerism." 



Fig. 3. — Older embryo showing metameric segments in the head-plate. The neural folds of the head 

 are nearly in the horizontal plane, op, beginning of the optic vesicle. 



Fig. 3 shows an older embryo with a slender trunk and 

 broadly expanded head. The optic vesicles {op) have made 

 their appearance on the head-plate. The neural segments are 

 well shown on the left-hand margin of the cephalic plate. 



Fig. 4 shows a still older embryo in which the neural folds 

 of the head have grown upwards to form an open neural groove. 



Fig. 4. — Embryo with open neural groove. 1-13 metameres of the neural folds, op, primary optic 

 vesicle, he. head-cavity. 



