Arthur B. Lamb 



189 



S. Obl. 

 Cil. S-. 



C. St. 

 Op. V. 



SI. 



from the undifferentiated mass of cells in which the notochord anter- 

 iorly ends. 



In Acanthias, as was shown above in the history of the anterior 

 somite, tlie down-growing infnndihulum divides the anterior prolonga- 

 tion of the archenteron into an anterior and posterior portion. From 

 the anterior portion the anterior somite develops. The posterior por- 



FiG. 3. — Reconstruction of optic vesicle, head somites and nerves of au Acau- 

 thias embrj'o, 16 mm. total length. Right side, medial view. 



tion, growing laterally, gives rise to the first somite of either side, 

 while axially it forms the connecting stalk or canal so characteristic of 

 this somite (Fig. 3, C. si.). At a stage when 31-22 somites are differ- 

 entiated, both the somite and the connecting strand are solid. This is 

 still the case at a 29-30 mm. stage, except that in the connecting stalk a 

 small cavity is visible. At a slightly later stage the number of these 

 median cavities has increased. Miss Piatt, gi, homologised these with 

 the median cavities described by Dohrn^ go, in Torpedo. 



