{( oiilnumlj'nnn piuji HI) 



roof l)l!iti-. in (■..iisrciucncc of which I lie (h)r,sal tii)s of the Lateral ph'ilcs arc widely 

 scpai-iitoil. 'IMhtc is a marked increase in (he nuinl >er uf nerve lil.ersin I he hateral 

 l)lates, especially in I he median and \-eidral portions. Of prime inii)ortance is 

 the great expansion of the lonrth \-entricle and the roof plate, which apparently 

 in Petroinyzoii can he explained only from inlcrnal factors, the mosl obvious 

 of wliich is the meidianical expansion due to an increase in the cercsbro-spinai 

 Huid. It will he seen that these forees were sufficiently strong to more than 

 offset the thickeninfi; of the hvteral plates which would tend to obliterate the 

 dorsal i)ortion of the embryonic central canal as it has the ventral portion. It 

 is apparent that this internal pressure has pushed the lateral wall apart in the 

 dorsal region, where the lateral plates are thinnest and weakest. As was pointed 

 out in the 20 day series the ependymal cells are becoming difTerentiated and 

 probably ha\-e assumed a secretory function. Likewise the increase in the num- 

 i)er of blood vessels above the roof plate favors filtration and diiTusion into the 

 fourth ventricle. X 125. 



.54 (See preceding plate.) Median longitudinal section through the head 

 region of a 20 day Petromyzon embryo introduced for a comparison w^ith the 

 transverse sections in figures 50 to 53. Note especially that the marked convexity 

 of the roof plate of the fourth ventricle is suggestive of expansion from an in- 

 crease of cerebro-spinal fluid. Absolutely no pontine fiexure is to be seen, the 

 little convexity that occurs in the floor plate can easily be attributed to an in- 

 crease in the number of nerve fibers. Observe that the fourth ventricle (CC.) 

 is the remains of the dorsal portion of the original embryonic central canal, while 

 the central canal of the spinal cord is the remains of the ventral portion. The 

 ventral portion of the embryonic central canal of the medulla has been obliter- 

 ated through the fusion of the ventral portions of the lateral plates. X 38.3. 



ABBREVIATIONS 



Aud.W, auditory vesicle or otocyst Myo-, myotomes 



B.V., blood vessel N.C., nerve cell 



C.C., central canal R.Ex., roof plate expansion 



C.C.C, central canal closure, caused R.P., roof plate of the central nervous 



by fusion of lateral plates system 



Ep., ependyma V.G., Gasserian or semilunar ganglion 



Ep.N., layer of ependymal nuclei VIII. VII., acustico-fascialis ganglion 



G.C., germinal cell W.M., white matter 



G.M., white matter X.G.. vagus ganglion (nodosum). 

 M.L., mantle layer 



PLATE 10 



EXPLANATION OF FIGURES 



55 to 62 Taken from various transverse sections through embryonic and larval 

 Polistotrema and Entosphenus (Pacific coast lamprey). Introduced to show the 

 effect of the developing notochord on the spinal cord in Cyclostomes. They 

 were drawn with the aid of an Edinger-Leitz drawing apparatus and reduced 

 one-half in reproduction. 



55 Transverse section through the caudal region of a 20 mm. Polistotrema 

 embryo. It will be seen at this stage that the notochord has produced very 

 little visible effect on the spinal cord. Cyclostome embryos of this stage (com- 

 pare fig. 39 for Petromyzon) present a nearly cylindrical spinal cord; while that 

 of all other vertebrates is more or less elliptical in cross section, the greater 

 (Continued on page 64) 

 62 



