EMBRYOLOGY OF THE SEA LAMPREY 



125 



as the auricle moves dorsal to the ventricle. Botli 

 auricular and ventricular walls begin to thicken 

 but remain transparent. The rate of heartbeat in- 

 creases from the 40 per minute of stage 14 to 100 

 per minute at the end of stage 15. 



Between the 13th and the 14th days a graj-ish 

 channel develops along the midventral line of the 

 cylindrical yolk mass. The channel extends from 

 the anterior portion to the midpoint of the yolk- 

 filled gut and within a short time turns red as 

 hemoglobin is produced. During the 15th day 

 bilateral channels appear along the midlateral line 

 of the yolk mass and then converge to form a single 

 vessel to the heart. 



Stage 15 is initiated by the appearance of a pair 

 of dorsal melanophores bilateral to the midbrain. 

 vSecond and third pairs appear in sequence immedi- 

 ately posterior to the original ones. Shortly after 

 the appearance of the third pair, a melanophore 

 can be seen above the anterior branchial region. 

 Pigmentation spreads anteriorly and posteriorly 

 from the dorsal pau-s along the neural tube (fig. 

 23) ; it spreads posteriorly and ventrally from the 

 lateral pair along the line between the yolk and the 

 somites, as far as the anterior limit of opaque or 

 undifferentiated tissue. As transparency pro- 

 gresses, the posterior distribution of melanophores 

 is extended correspondingly. 



The end-point of stage 15 is reached with the 

 appearance of the gill slits. 



Stage 16: Gill-cleft (fig. 24) days 15-17 



Size: 6 to 7.5 millimeters. 



Neural tissue: Divisions of central nervous 

 system recognized through ectoderm. 



Gut: Cj'lindrical yolk mass. Increases in 

 length and becomes more slender. No ventral 

 flexion. Anterior portion greenish. Postanal gut 

 present. Anus forms at persistent blastopore. 



Mouth: Semicircular slit. Enlarges to hooded 

 mouth. Oral cirri make first appearance. 



Somites: No longer useful as staging criterion. 



Locomotion: Larval swimming pattern. Very 

 adept at end of stage. 



Nostril: Migrates from anteroventral to antero- 

 dorsal extremity. 



Transparency: Practically entire prolarva be- 

 comes transparent except gut. 



Circulatory system: Ventricular wall has thick- 

 ened. Heart beat, 150 per minute. Flow of 

 blood readily visible. 



Liver: Size increases. Distinctly separated 

 from gut. 



Pigmentation: Melanophores extend along dor- 

 sum and sides of embr^'o from anterior to posterior. 

 Bilateral eyespots appear at end of stage. 



Gill clefts: First appearance. They become 

 functional. 



Respiratory rate : 120 per minute. 



Fins: Caudal and anal fins appear. 



The mouth continues to migrate anteriorly and 



Figure 23. — Dorsal view of stage 15 showing condition of pigmentation over the head region. 



Figure 24. — Lateral view of stage 16, gill cleft, showing gill clefts, somites, pericardial area, and pigmentation 



over yolk mass. 



