SPINAL CORD AND MEDILLA OF CYCLOSTOMES 33 



the nearly cylindrical spinal cords of the Cyclostomes (Petro- 

 myzon and Polistotrema) to the very much compressed (flat- 

 tened laterally) cords of the chick and pig. Were it not for the 

 intermediate stages of Amblystoma and the turtle we might bo 

 justified in establishing two distinct types of the embryonic 

 spinal cord: t\i)o 1 cylindrical, and type 2 compressed. We 

 could even go fuithor and classify the Cyclostome embryonic 

 cord under type 1 and the Gnathostome cord under type 2. 



While the equal expansion of the spinal cord through the addi- 

 tion of nerve cells and fibers to the typical embryonic stage in 

 the pig would tend to produce a cylindrical spinal cord, and in 

 the C'3'clostomo would tend to produce a depressed cord, the 

 internal structure shows that the origin of the differences between 

 the spinal cord of a Cyclostome and a mammal is not so simple. 

 The neural axis of Pctiomyzon in an early stage, corresponding 

 to stage fa) above, is decidedly c()mi)ressed instead of depressed. 

 The problem then is to explain the change from a laterally com- 

 pressed cord in the early IVtromyzon embryos to the gradunllx 

 depressed, ribbon-like. si)inal cord of the adult. 



A careful examination of figures of) to .')". wliicli are taken from 

 practically the same region from three dilYerent Polistotrema 

 embryos, shows \-ery elcMi-ly that the growing notoeliord is 

 bringing about the marked flattening (depression) and ventral 

 indenting of the spinal cord. Tn connection with figure 5") it 

 should be noted that tlie spinal cord is in the so-called typical 

 embryonic stage, and amid siuToundings peculiarly favorable for 

 undergoing a depression from a rapidly growing notochord. It 

 will be seen that the spinal cord is closely enveloped by a men- 

 ingeal membrane (P.M.), approximating a layer of connective 

 tissue, the future^ membranous neural arch, which is firmlj^ at- 

 tached below to the growing notochord. Directly above, the 

 mesenchyme is proliferating and migrating toward the center 

 to form a median dorsal cartilage (M.D.C.) ; while there is appar- 

 ently but little lateral resistance in the way of massing of mesen- 

 chyme and the formation of myotomes. In a somewhat later 

 stage (fig. 56) some growth has taken place in the notochord. 

 producing a slight indentation on the spinal cord. In a much 



THE JOrRNAl. OK CO.MI'AH\TI\ K NElROI.UtiY, VOL. 26, NO. 1 



