48o 



WILSON. 



PLATE XX. 



[All of the figures from preparations. Prototrochal pigment, and the cilia, from 

 living specimens.] 



Fig. 83, Ventral view. Ingrovs^th and concrescence of the neural plates, separa- 

 tion of the stomodaeum and pigment-area; first appearance of metamerism (24 h.). 



Fig. 84. Right side-view of the same specimen (slightly distorted by pressure). 



Fig. 85. Later stage, right side-view. Formation of the setae, longitudinal mus- 

 cles, parapodial cirrhi (40 h.). 



Fig. 86. The same specimen from the upper pole. First and second cleavage- 

 lines, cross-furrow, eye-spots, frontal bodies. Cf. the cleavage-stages. 



Fig. 87. The same specimen, ventral view. Concrescence of the neural plates, 

 growth of the ventral region, overlapping of the head-kidneys (40 h.) . 



Fig. 88. Actual longitudinal section of a stage like Fig. 78. Relations of the 

 ventral plate, stomodseal cells, primary mesoblasts, entoblast-nuclei, and the inward 

 migrating cells of the pigment-area with their pigment (14 h.). 



Fig. 89. Oral view of early stage in the elongation. Protrusion of the setae 



(43 h.)- 



Fig. 90. Protrusion of the parapodial cirrhi. Apical and anal sense organs, 

 migration of the pigment-cells (60 h.). 



Fig. 91. Typical three-segmented larva, dorsal view. Completed migration of 

 the pigment-cells, appearance of the mesotrochal cihated belts, circular muscles, 

 three pairs of eye-spots, first pair of tentacular cirrhi (post-trochal), and anal cirrhi 

 (60 h.). 



Fig. 92. Head of later stage (4^ d.) in ventral view. Demarcation of the head, 

 appearance of the jaws, growth of the frontal antennse, tentacular cirrhi, palpi, etc. 

 Interruption of the prototroch. 



