12 BIOLOGICAL SURVEY OF 



differentiated brain vesicles, pigmentation of the abdominal viscera, 

 the large thin flexible pectoral fins, and the long oar-like pelvic fins. 

 Larvae 5.6 to 6.2 mm. in length. 



July 12th, thirteenth day of development (fig. 22). Temperature, 

 3 P.M., 19.0°C. 

 Little advance over the preceding day ; pigmentation somewhat 

 reduced ; body and head slightly slimmer ; these changes may be 

 the expression of reduced metabolism. The larvae have decreased 

 slightly in length since the ninth day (July 8th). At this time only 

 a dozen or so specimens remained, and the next day all were dead. 



Comparison of these specimens with the development of 

 larvae from PljTuouth, England, as figured by Marie Lebour 

 ('25), shows apparently unimportant differences. The Eng- 

 lish specimens were reared in the laboratory for twelve days ; 

 ours, thirteen days; but since the English specimens when 

 captured were hatched, whereas ours were only a few hours 

 after fertilization, our final stages correspond in time to the 

 eight- or nine-day stage of the English series (figs. 3c and 

 4a, in the paper referred to above). The drawings of the 

 embryos figured show our final stage (thirteenth day) to be 

 slightly less advanced in structure (third dorsal ray not 

 present, pelvic fins relatively shorter) than the eight-day 

 English specimen (fig. 3c). The water temperature of the 

 English specimens averaged about a degree higher. The 

 rate of development of the English specimens and ours 

 shows as close a degree of correspondence as could be 

 expected. 



The most nearly constant difference appears to be the 

 pigmentation of the posterior part of the body. Practically 

 all our specimens after the fifth day (July 4th, fig. 11) have 

 two and usually three pigmented areas, as shown in the 

 figures; the most posterior spot is practically constant. On 

 the other hand, Marie Lebour figures two ventral pigment 

 spots in figure 3b of her paper (seventh day, corresponding 

 to about the fifth day of our series) and one dorsal spot in 

 figure 3c (eighth day of her series). Miss Lebour remarks 

 that the pigmentation is not constant in her specimens 

 (p. 726) ; also the pigmentation on the pelvic fin is more 

 diffuse in the English specimens than in ours. 



