422 



WILSON. [Vol. VI. 



full activity. The approximate outlines of the neural plate are 

 shown by the dotted lines in Fig. Z6, but there is no distinct 

 boundary, the neural plate being indicated only by the greater 

 depth of the cells and by their smaller size. 



The head-kidneys now completely surround the body in the 

 position shown in Fig. 82. 



At this period the larvae swim very actively, in the usual 

 manner, rotating about the vertical axis and exhibiting a very 

 marked heliotropism. 



From this time forward I shall employ a different set of terms 

 in the orientation (see p. 426 for the grounds on which this pro- 

 cedure is based). I shall designate the site of the frontal bodies 

 as the anterior extremity, the centre of the pigment-area as the 

 posterior, the flattened (anterior) side of the lower hemisphere 

 as ventral, and the opposite side as the dorsal. 



The general character of the metamorphosis is shown in PI. 

 XX. The body gradually elongates in an axis that coincides 

 with the original vertical axis {i.e. at right angles to the proto- 

 troch), the pigment-area being carried away from the mouth 

 and remaining at the posterior end of the body. Meanwhile 

 the cells of the ventral plate grow inwards from each side 

 towards the middle line between the pigment-area and the 

 stomod^um, which are thus forced apart ; and in the space thus 

 formed the two halves of the ventral plate undergo a regular 

 process of concrescence along the median ventral line, where 

 a distinct seam may for some time be seen in the surface-views 

 (Figs. Z^, ^7). The origin of the median ventral region may 

 thus be followed step by step — almost cell by cell — from the 

 beginning. On either side the line of concrescence appears a 

 distinct thickening, the ventral neural plate, from which arises 

 the ventral nerve-cord. Thus the ventral cord arises from two 

 separate halves which have at first no connection with the 

 cephalic neural plate. 



As the elongation begins, the seta-sacs make their appear- 

 ance, and with them is given the first indication of metamerism. 

 Those of the first two somites appear first (Figs. 83, 84), the 

 dorsal and ventral rami being represented each by a somewhat 

 vague rounded mass of cells. Those of the third somite appear 

 somewhat later (Fig. 85), about the same time with the differ- 

 entiation of the lonofitudinal muscles. At the same time the 



