OP ARTS AND SCIENCES. 19 



PLATE in. 



Pleuronectes Americanus Walb. 



Platessa plana Storer PI. XXX. fig. 2. 



Fig. 1. Young, about 4 mm long a few days after hatching. Seen from the left 

 side. The eyes are symmetrically placed at the extremities of an 

 axis at right angles to the longitudinal axis. The pectorals are well 

 developed, the embryonic fin extends unbroken from the base of 

 the brain to the anus, the ventral portion is somewhat broader. 

 The eyes are of a light bright-green, and there are faint yellow 

 patches on the lower sides of notochord along the muscular bands. 



„ 2. Somewhat older than fig. 1. The tail has become slightly hetero- 

 cercal, and the embryo is much less transparent than in the previous 

 stage. The muscular tissue above and below the notochord is of 

 a light-brown color, with yellow patches near the black pigment 

 spots. One or two very indistinct tail-rays have begun to form. 



„ 3. In this stage, the principal changes are confined to the increased num- 

 ber of tail fin-rays, and to the segmentation of the vertebral column 

 sending out its dorsal and ventral cartilaginous apophyses. The 

 pigment spots of the embryonic fin-fold (fig. 1), as well as of other 

 parts of the body, seem to become more prominent, when increased 

 activity in the formation of new tissues takes place. See the pig- 

 ment spots in the tail of this figure. 



„ 4. A somewhat more advanced stage, in which the dorsal and ventral 

 embryonic fold has become tolerably separated from the tail-fin. 

 At the base of the dorsal and ventral folds, the basal fin-rays are 

 well developed, but as yet we find no trace of the fin-rays proper. 



„ 5. In this stage, the tail-fin is in great part separated from the embryonic 

 fin-fold, which shows here and there traces of the formation of the 

 fin-rays proper ; but in other respects it differs from the preceding 

 stage mainly in the greater number of pigment spot patches, in 

 the greater development of the muscular bands, and of the dorsal 

 and ventral apophyses of the vertebral column. The eyes are as 

 yet symmetrical. The length of this embryo is about that of the 

 preceding stage (fig. 4). 



PLATE IV. 



Pleuronectes Americanus Walb. 



Pig. 1. We now come to a series of stages in which the body becomes broader 

 in proportion to the length, and in which the dorsal and anal fins 

 are all gradually isolated from the caudal. In this stage, the fin- 

 rays extend nearly to the edge of the dorsal and anal, the muscu- 

 lar bands are much wider, and there is a slight asymmetry in the 

 position of the left eye, which has moved well forward towards 

 the top of the snout ; while in the preceding stages the left barely 



