82 WILLIAM F. ALLEN 



CAUSES UNDKRLYIiNG THE FLATTENING OF THE SPINAL 

 CORD IN CYCLOSTOMES 



If our attention is first directed to a transverse section through 

 an adult Polistotrema (figs. 10 and 71) it might be inferred, since 

 there is ample room in the membranous neural canal for a well- 

 rounded spinal cord, that the flattening of the spinal cord might 

 be attributed entirely to internal factors. A glance at a trans- 

 verse section through a developing Polistotrema spinal cord 

 (fig. 57) suffices to show that there is proportionately much less 

 room within the membranous neural canal, and that a mechani- 

 cal force in the form of a rapidly growing notochord is at work 

 immediately below the spinal cord. 



At the outset it seems advisable to establish arbitrarily a typi- 

 cal state of an embryonic spinal cord, by which a direct com- 

 parison of one form can be made with another. The examina- 

 tion of very early stages of the spinal cord in a large number of 

 embryos of Squalus, Amblystoma, the chick, and the pig, in 

 all of which the neural tube is formed by the rolling up of the 

 neural plate, shows that the neural tube passes through three 

 stages: a) A depressed tube with the central canal in the form 

 of a horizontal cleft; b) a cylindrical tube with the canal circu- 

 lar in cross section; and c) a laterally compressed tube with the 

 canal in the form of a vertical cleft (figs. 81 to 86). The exist- 

 ence of this series of changes in Squalus has been shown in a 

 table of developmental stages compiled by Scammon. The third 

 stage may be selected as the typical embryonic spinal cord. 

 This stage is reached at about the time of the first appearance 

 of nerve fibers in the marginal layer. 



As a result of a comparison of the typical embryonic stages of 

 the spinal cord in the following transverse sections^ (fig. 39 for 

 Petromyzon, fig. 55 for Polistotrema, fig. 81 for Squalus, fig. 



83 for Amblystoma, fig. 84 for the turtle, fig. 85 for the chick, 

 and fig. 86 for the pig) it is clear that we have all gradations from 



* It is obvious that this comparison would have no value unless the sections 

 were truly transverse sections. To avoid selecting oblique transverse sections, 

 these figures were always drawn from anterior trunk sections if the embryo 

 showed any flexures. 



