I'LAri'i II 



KXl'I.WATION OK I'KIKKKS' 



7'J III SO ic|)ics('rit M nuiiilxTor I r;msvcrs(> so(^t ions through the medulla of shark, 

 iunphihian, and pi-^ cndtryos lor tlic purpose of demonstrating various stages of 

 roof plate expansion. 



72 leather ohliciue transverse seel ion tlirout^h tlic medulla of a 19 mm. Squa- 

 lus emhryo in the region of the \ni ganglion (from series No. 2 of Professor 

 Seainmon's colleet ion) . Note the well-formed fourth ventricle, and the Ijroadly 

 expanded and very mueh stretched roof plate. Its collapsed appearance in this 

 section is doubtless due to fixation or preparation. On account of a great pro- 

 liferation of cells and nerve fibers the lateral plates have fused ventrally, as 

 in Petromyzon, obliterating that part of the embryonic central canal. Attention 

 should be called to the fact that the medulla roof plate in sharks begins to expand 

 much earlier than it does in Petromyzon. This figure shows a well-expanded 

 roof plate, while the cells in the mantle layer are no more differentiated and there 

 are no more nerve fibers in the marginal layer than appear in a 12 day Petromy- 

 zon embryo (fig. 40), where there is no fourth ventricle and no expansion of the 

 roof plate. X 70. 



73 Transverse section through the medulla of a 15 mm. Necturus taken 

 through the VIII ganglion. Observe the wide fourth ventricle and the broadly 

 expanded and greatly stretched roof plate, and the coagulated appearance of 

 the cerebro-spinal fluid in the ventricle. Like Squalus, the fourth ventricle 

 begins relatively much earlier than in Petromyzon. It should be noted that no 

 blood vessels have reached the level of the roof plate or entered the medulla; 

 hence the coagulum in the ventricle must be largely a product of secretion. X 39. 



74 From a transverse section of a very young Amblystoma embryo in the 

 region of the auditory vesicle (Professor Johnston's series No. 50). Here there 

 has occurred a dorsal and a smaller ventral excavation of the cleft-like central 

 canal. It will be seen that the larger dorsal cavity, the beginning of the fourth 

 ventricle, possesses no thinner roof plate than does the spinal cord (fig. 83). 

 Also in this section (fig. 74) the roof and floor plates are about equally thick. 

 What has taken place dorsally and ventrally throughout the embryonic central 

 canal has been a migration of the cells outward. The fact that in this section 

 of the medulla the roof plate is no thinner than in the section of the spinal cord 

 (fig. 83), taking note that the spinal and cranial ganglia are well-formed, is evi- 

 dence against the hypothesis, that the greater migration of the neural crest 

 cells of the medulla was the prime cause of the thinning out of the roof plate of 

 the rhombic brain. X 70. 



(Continued on page llf) 



ABBREVIATIONS 



Aud.V., auditory vesicle or otocyst M.L., mantle layer 



B.V., blood vessel Myo., myotomes 



C.C, central canal or cast of the same N.A., membranous neural arch 



C.C.C., central canal closure, caused N.C., nerve cell 



by fusion of lateral plates A^c, notochord 



Ep., ependyma P.O., pigmented or eye cells of Amphi- 



Ep.N., layer of ependymal nuclei oxus 



P.P., floor plate of the central nervous R.Ex., roof plate expansion 



system R.P., roof plate of the central nervous 



G.C., germinal cell system 



L.S., lateral veno-lymphatic sinus or S.C.F., cerebro-spinal fluid 



anlage of the same V.G., Gasserian or semilunar ganglion 



Mar.L., marginal layer VIII.G., Auditory ganglion 



M'.C, Miillerian or giant cells W.M., white matter 



M.F., Miillerian or giant fiber 4 V., fourth ventricle 



72 



