130 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



they loosely cover the dorsum and form a hne 

 posteriorly from the branchial region between 

 yolk and somites. In stage 16 melanophores 

 extend along the dorsum and sides of the embryo. 

 Bilateral eyespots appear at the end of stage 16. 

 The melanophores become more extensively dis- 

 tributed in stage 17. Aggregations appear over 

 the pronephric region and the caudal end of the 

 notochord and neural tube. The anterior lip, 

 and the head region and the gut become profusely 

 covered with pigmentation. The pattern of pig- 

 mentation changes little in stage 18 except that 

 the pigmented area is extended. 



Activity in the embryo included locomotory 

 movements, heart beat, and velum beat. First 

 evidence of activity is the movement of the head 

 region, produced by muscles of somites of stage 13 

 embryos. Greater muscular movements take 

 place during later stages, including the movements 

 that lead to : hatching; poor swimming movements 

 of stage 14; somewhat better swimming in stage 

 15; the greatly increased deftness of swimming in 

 stage 16; and movements which accomplish bur- 

 rowing at stage 17. 



Changes in the heart region in stage 14 embryos 

 can be seen through the ectoderm over the peri- 

 cardial cavity as it becomes transparent. The 

 heart begins to beat at a rate of 40 times per 

 minute, increasing to 100 per minute in stage 15, 

 150 per minute in stage 16, and more than 200 

 per minute in stage 17. 



The respiratory system becomes functional at 

 stage 16 when the initial respiratory rate (velum 

 beat) is 120 per minute. This rate increases to 

 200 per minute in stage 17 and is maintained at 

 this rate into stage 18. 



In general, lamprey development resembles 

 amphibian development in cleavage (stages 1-8). 

 Lamprey gastrulation resembles amphibian gastru- 

 lation in some respects and teleostean in others. 

 Epiboly is more like that of amphibians than of 

 teleosteans. Invagination and involution of 

 chorda-mesoderm seem similar to that in amphib- 

 ians. Formation of the lamprey archenteron 

 produces a situation comparable to that of a 

 teleost. In both, the developing embrvo is lo- 

 cated on a large yolk mass but separated from the 

 yolk by an archenteron which was the original 

 blastocoel. A major diflFerence between the two is 

 that lamprey yolk is divided whereas teleostean 

 yolk remains undivided. Another similarity to 



teleostean development is the formation of a solid 

 neural rod which develops a lumen only after 

 neurulation has been completed. 



DEVELOPMENT AT DIFFERENT 

 CONSTANT TEMPERATURES 



The primary objective of the experimental 

 rearing of sea lampreys at a series of constant 

 temperatures was to determine the temperature 

 levels at which the eggs were capable of developing 

 into normal, viable larvae. As part of the work, 

 detailed records were kept on : the relation between 

 temperature and progression of development; 

 mortality rate during development ; occurrence and 

 nature of developmental abnormalities. Infor- 

 mation of this type may help explain the failure, 

 noted both in the LTnited States and in Canada, 

 of the sea lamprey to utilize certain apparently 

 suitable spawning streams as extensively as other 

 apparently similar waters. 



MATERIALS AND METHODS 



The experiments were conducted at tempera- 

 tures and with the aid of equipment indicated in 

 table 2. Throughout this work, control lots at 

 65° F. were maintained as an index to develop- 

 ment. Thus, any one control lot could serve 

 efTectively as an indicator to several other experi- 

 ments. Eggs from 2 to 4 females were mixed with 

 sperm from 4 to 8 males. These eggs were then 

 apportioned into containers in the numbers (by 

 actual count) indicated in table 3. 



Table 2. — Equipment used in experiments on development 

 at constant temperatures 



The sampling schedule differed somewhat among 

 the experiments (as may be seen from later tables 

 that give details for individual samples) but the 

 differences of schedule and the unavoidable occa- 

 sional interruptions of timing were not sufficient 

 to impair comparisons between series or to hamper 

 the description of the progress of development. 

 In the main, the earlier samples were taken at 

 1- or 2-hour intervals; the time between samples 



