EMBRYOLOGY OF THE SEA LAMPREY 



129 



the vegetal cells are covered by the epibolizing 

 animal colls. 



Blastopore formation, as was observed by 

 Shipley (1885), involves behavior of animal cells 

 and vegetal cells similar to that in amphibians and 

 teleosts. When invagination of animal cells 

 forms the blastopore, the margin of animal cells 

 is arranged latitudinally around the embryo from 

 the blastopore. As the animal cells continue to 

 epibolize, the vegetal cells are covered by animal 

 cells from the anterior and dorsolateral portions 

 of the animal hemisphere and subsequently from 

 the posterolateral and ventral positions in the 

 fashion described for stage 9. Thus, migration of 

 animal cells over vegetal cells in the lamprey 

 matches closely the epiboly of animal cells in 

 amphibians. The dorsal lip does not curve as 

 much, however, in the sea lamprey as in amphib- 

 ians. If the blastopore is considered to be de- 

 fined by the line of epibolizing animal cells, as it is 

 in teleosts, the blastopore of the sea lamprey is at 

 first oval. This shape results from alignment of 

 animal cells lateral to the midsagittal plane. A 

 circular blastopore is formed when animal cells 

 at the ventralmost point of the oval-shaped 

 blastopore cover the oval yolk plug in a ventral- 

 to-dorsal direction along the mid-sagittal plane. 



The circular blastopore of the sea lamprey 

 apparently migrates toward the center of the 

 animal hemisphere whereas the teleost blastopore 

 migrates toward the vegetative pole as the embryo 

 lengthens. The amphibian blastopore eventually 

 reaches and passes beyond the vegetative pole at a 

 stage equivalent to stage 13 of the lamprey. 

 Because teleost embryos do not extend ventrally 

 over the yolk mass the blastopore does not reach 

 the vegetative pole. 



The apparent migration of the lamprey blasto- 

 pore might possibly be attributed to the decrease 

 in the vohim(> of the blastocoel during stages 9 

 and 10. An increase in embryo volume which 

 takes place in stage 8 is apparently due primarily 

 to the incr(>ased volume of the blastocoel. During 

 stage 9, after involution is completed, the volume 

 of the eml)ryo decreases as the result of a decrease 

 in th(> volume of the archenteron. The archen- 

 teron, in reality, is the original blastocoel since 

 th(> blastocoel is not obliterated during gastrula- 

 tion (histological observations) as it is in amphib- 

 ians. In this feature of gastrulation the lamprey 

 closelv resembles teleosts. Since decrease in 



the size of the archenteron shortens the embr\'0 and 

 since growth of the neural tube in stages 12 and 

 13 moves the blastopore along the mid-sagittal 

 plane, the blastopore appears to migrate. 



The present study also corroborates Shipley's 

 (1885) observation that the open blastopore 

 persists, eventually becoming the anus. Histo- 

 logical sections permit tracing the archenteron 

 to an open blastopore through stage 15. During 

 stage 16, the diameter of the blastopore widens 

 and the lips thicken to form the anus. The 

 archenteron can be traced to the anus in stage 16 

 as it was traced to the blastopore in stage 15. 



Shipley recognized that the early neural tube 

 did not possess a neurocoel and called it a neural 

 rod, which term has been retained here. Selys- 

 Longchamp (1910) described neural-tube forma- 

 tion in lampreys as intermediate between the keel 

 method of teleosts and neural-fold method of 

 other vertebrates. 



Among the morphological features promuient 

 during development were the gut, liver, gall 

 bladder, hemoglobin and vascularization, and 

 pigment. The gut opens from the oral cavity to 

 the anus at stage 18. The stomodaeum opens in 

 stage 16, the esophagus in stage 17, the cloaca in 

 stage 17, and finally the portion between esophagus 

 and cloaca at stage 18. 



The formation of the hver in late stage 14 is 

 indicated by the greenish cast of the anteriormost 

 portion of the gut. Vascularization of the liver 

 occurs during stage 15. The size of the liver con- 

 tinues to increase through stages 15 and 16. A 

 gall bladder forms in stage 17; it is recognizable 

 externally bj^ its accumulation of blue-green bile. 



Additional changes in stage 15 embryos include 

 the appearance of hemoglobin within the blood 

 channels which had formed in the yolk-filled gut. 

 Blood cells appear first in the midvcntral channel 

 and soon are in all three major channels. Blood 

 formation and the bulk of vascularization take 

 place in stages 15 and 16. Vascularization is ex- 

 tended in stage 17, when practically all major 

 vessels can be traced by following red corpuscles 

 within the transparent tissues. 



Pigmentation of the embryo begins as two 

 melanophores bilateral to the midbrain. Succes- 

 sive pairs of melanophores appear posterior to the 

 initial one; n(>xt is the appearance of a melanophore 

 above the branchial region. The number of 

 melanophores increases throughout stage 15 when 



