EMBRYOLOGY OF THE SEA LAMPREY 



141 



that had reached stage 6 (32 cells) but had begun 

 to vacuolate. All subsequent samples had oid}' 

 dead embryos. 



Table 23. — Living and dead embryos per sample and their 

 stage of development at 80° F. (26.6° C.) 



Table 24. — Mortalities of sea lamprey eggs reared at a 

 constant temperature of 80° F. '{26.6° C.) 



1 Only 2 live embryos were taken after 11 hours and all (914) were dead 

 after 15 hours. 



SIGNIFICANCE OF OBSERVATIONS 



Effect of temperature on development 



The most significant result of the experimental 

 rearing of sea lamprej' eggs at 10 difTerent constant 

 temperatures (ranging from a minimum of 45° 

 F. to a ma.ximum of 80° F.) was the clear demon- 

 stration that successful development through to 

 the production of viable burrowing larvae was 

 possible only within a relatively narrow range. 

 No live larvae were produced at any temperature 

 below 60° F. or above 70° F. Further evidence 

 of the extreme sensitivity of sea lamprey eggs to 

 temperature comes from the much lower survival 

 at 60° F. (12 percent), and 70° F. (5 percent), 

 than occurs at the "optimum" temperature of 65° 

 F. (78 percent). It is to be regretted that experi- 

 ments were not made at 62.5° F. and 67.5° F. to 

 define more clearly the trends within the 60°-70° 

 range, but no further time was available when 

 the importance of tests at these two intermediate 

 temperatures became obvious. 



Mortality was so heavy at the highest and lowest 

 temperatures that all eggs had died before devel- 

 opment had proceeded beyond very early stages. 

 In general, the highest stage reached increased as 

 the temperature approached the "successful" 



levels of 60°-70° F. This relationship is brought 

 out by the following listing: 



Temperature (,F.) Highest stage reached 



45°. 3 (4 cells) 



50° _ 9 (gastrula) 



52.5° 12 (head) 



55° 13 (prehatching) 



60° 17 (burrowing) 



65° 17 (burrowing) 



70° 17 (burrowing) 



75° 16 (gill clefts) 



77.5° 6 (32 cells) 



80° 6 (32 cells) 



Developmental abnormalities were least plenti- 

 ful at 65° F. and increased as the temperature 

 deviated from that value in either direction. In 

 some tests the incidence was high, and the abnor- 

 malities (described briefly in the accounts of the 

 experiments) involved monstrous distortions of 

 the embryos. 



In general, developmental rate (notably, the 

 time required to reach the various stages) became 

 faster, lengths of stages became shorter, and over- 

 lap betw^een stages was lessened as temperature 

 increased. Some of the exceptions to this state- 

 ment no doubt represent the random variability 

 of the data. Others, as for example, the seem- 

 ingly depressing effect of the highest temperatures 

 on the rate of development in the early stages may 

 reflect a real cause-and-effect relationship. 



The clear demonstration in the present studies 

 that sea lamprey eggs are capable of full and nor- 

 mal development only within a relatively narrow- 

 temperature range brings out the great importance 

 of controlling temperature at the correct level in 

 developmental studies and experimental research. 

 Consideration of the proper temperature had little 

 place, nevertheless, in past studies of sea lamprey 

 eggs. Authors failed to state the temperatures 

 at which the eggs were reared or reared them at 

 levels at which full, normal development could not 

 be expected. Shipley (1885) did not state the 

 temperature at which his sea lamprey embryos 

 developed and McClure (1893) reared his embryos 

 at 6°-7° C. (42.8°-44.6° F.). Damas (1948) men- 

 tioned temperatures of 12° and 18° C. (53.6° and 

 64.4° F.) on the development of Lampetra (it is 

 not to be assumed, of course, that the effects of 

 temperature on development are the same for 

 Petromyzon and Lampetra, but certain parallels 

 must be considered highly probable). 



