THE DEVELOPMENT OF THE DORSAL VENTRIC- 

 ULAR RIDGE IN TURTLES' 



J. B. JOIIXSTOX 

 Universili/ of Minnesota 



TWE.VTY-SK\ KV FKilKKS 



In tlic adult tuitlc (Jolinstdii. "l.")!)) the lar^c ridge project- 

 ing into the ventiiclc from \\\o iaicial wall of the hemisphere 

 has strong fiber conncclion-- with tlie cerebral ])edunele and 

 has obvious close relations with the cortical layers of the pallium 

 m the caudal ])ole. Indeed, in its caudal piirt this ridge a])pears 

 to be a fold of tlic whole thickness of the brain wall and is 

 marked externally by tiie "amygdaloid fissure.' Here the cell 

 layers of the general pallium turn in to become continuous with 

 the cell masses of the ridge. It is evident from these facts that 

 the relations of the doi'sal vcntiiculai- ridge aic cliiefly with the 

 general ])allium. Its |)osition lateral and dorsal to both the 

 caudate and lentiform miclei shows that it does not belong 

 to the corpus striatum. Its independence from other cell 

 masses in its rostral part gi\'es one the impression (from the 

 study of adult material alone), that the ridge has been formed 

 by infolding from the caudal region ('lob, p. 417). 



The origin of this ridge in the embryo shoukl throw light 

 not only on its relations in the turtle but also on its signihcance 

 in the evolution of the manunalian brain. I am able to give 

 the main outlines of the development on the basis of a series 

 of early embryos of Chelydra serpentina, for which I am in- 

 debted to Dr. C. E. Johnson of this University; and of sec- 

 tions of 17, 20, and 28 mm. embryos of Chrysemys, belonging 

 to the Anatomical Laboratory of Washington L^niversity for 

 the loan of wliicli 1 am indebted to Dr. Edwin A. Baumgartner. 



I Neurological Studies, University of Minnesota. No. 23, July 1, 1916. 



481 



