EMBRYOLOGY OF THE SEA LAMPREY 



127 



Circulator}" system: Heart walls thicken. 

 Heart beat more than 200 per minute. Flow of 

 blood visually traceable throughout most of 

 prolarva. 



Liver: Further increases in size. First indica- 

 tion of gall bladder on right side. 



Pigmentation: Melanophores extensively dis- 

 tributed. Aggregation over pronephric region 

 and caudal end of notochord and neural tube. 



Respiratory rate: 200 per minute. 



Pronephros: First detectable externally. 



The posterior region of the gut becomes trans- 

 parent as the cloaca is formed with the opening 

 of the pronephric ducts into the gut (histological 

 observations). A yolk pellet forms in the gut ante- 

 rior to the cloaca as the result of peristaltic move- 

 ments of the gut. These movements move the 

 pellet toward the anus and then back to the 

 remaining yolk mass. 



The lips take on the characteristic larval form. 

 The mouth opens directly anteriorly, so that the 

 anterior lip has now become the dorsal lip and the 

 posterior lip the ventral. 



Stage 17 begins when the prolarvae burrow into 

 the bottom mud. Burrowing is the result of the 

 action of both the tail and the head regions. As 

 the head moves from side to side to create space 

 within the mud, the lashing of the tail drives the 

 prolarva into the mud. Prolarvae of this stage 

 placed in aquaria or beakers first swim near the 

 surface of the water and then suddenly plunge 

 downward with rapid swimming movements until 

 they reach the bottom, when they immediately 

 begin to burrow. Prolarvae of stage 16 merely 

 drift down to and lie on the bottom. Swimming 

 movements do not differ from those of stage 16. 



The liver is extended farther and the presence 

 of the gall bladder in the later period is marked 

 by its bile-green color. 



The eyespot or retina is very prominent anterior 

 and dorsal to the velum. Melanophores have 

 spread completely around the gut region to the 

 ventral surface. They have migrated down the 

 lateral surface of the velum and the gill bars, 

 and have completely outlined the branchial 

 basket. The dorsal lip is covered completely 

 with melanophores. 



A pronephros dorsal and posterior to either side of 

 the pericardial cavity is visible externally because 

 of the presence of much blood. This stage is the 

 first in which the pronephros is visible externally. 



The end-point of stage 17 is reached when the 

 lumen of the yolk-filled gut is opened. 



Stage 18: Larva (fig. 26) days 33-40 



Size: 9 millimeters and longer. 



Gut: Lumen completely opened. Yolk ex- 

 truded from gut. Gut tissue becomes trans- 

 parent. 



Respiratory rate: 200 per minute. 



Figure 26. — Various views of several stage 18 larvae on 

 a black background. Shown are: condition of the mouth 

 and lips, eyespot, pigment outlining branchial region, 

 liver immediately posterior to the heart, somites, fins, 

 and the gut from the liver to the cloaca. The granular 

 appearance along the gut is caused by pigmentation. 



The transition from the prolarval to the larval 

 condition (all systems differentiated save genital) 

 is marked by the differentiation of the formerly 

 yolk-filled gut into its definitive form. This 

 change is seen outwardly as the gut becomes 

 transparent at the "stomach" region behind the 

 liver. Transparency progresses posteriorly until 

 the entire gut has differentiated and the digestive 

 system is open from the mouth to the anus. 

 After the pellet described in stage 17 is passed, 

 yolk remaining within the lumen of the gut is 

 extruded constantly from the anus. 



Stage 18 larvae are between 9 and 10 milli- 

 meters long at the time the gut becomes fully 

 differentiated. They belong to the first larval 

 stage wliich would be equivalent to age-group 

 0, 11 to 21 millimeters long, of Applegate (1950). 



