AND OF LIGHT. 227 



suits obtained by O. Hertwig * upon the ova of Mana 

 fusca. The curves represent the number of days re- 

 quired by the ova, kept at different temperatures, to 

 reach certain definite stages. The ordinates indicate 

 the time in days after fertilisation, and the abscissae the 

 temperature. For the lowest curve the stage to be 

 reached was that of a gastrula with the blastopore clos- 

 ing in, and we gather from this curve that the time re- 

 quired at a temperature of 1 C. was 23 days; at a tem- 

 perature of 6, 4.9 days, and at a temperature of 24, 

 only a single day. Stage II was that of an embryo 

 having a rudimentary medullary plate, with its edges 

 rising and separated by a broad cleft; Stage III that 

 of an embryo with a closed medullary tube, and with a 

 distinctly marked head: Stage IV that of a more elon- 

 gated embryo with an obvious tail, but with gills not 

 formed; Stage V that of an embryo 5 mm. long, with 

 rudiments of gills; Stage VI that of an embryo 7.5 mm. 

 long, with well-developed gill tufts and tail 3.5 mm. 

 long; Stage YII that of an embryo 9 mm. long, with a 

 tail 5 mm. long. The curves representing the times of 

 growth to all these more advanced stages are very simi- 

 lar to each other and to the first curve. 



In the observations thus far quoted, the highest tem- 

 perature employed was 26, and this proved to be also 

 the most favourable temperature for growth. Higher 

 temperatures still may produce an adverse influence, as 

 we have already observed in the case of Echinoid larvae. 

 For the growth of tadpoles' tails, Lillie and Knowlton 

 found the " optimum " temperature to be 30, the rate 

 of increase in length then being 10.6 times greater than 

 *Arch. f. mik. Anat., li. p. 319, 1898. 



