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Robert J. Terry 



from a protoplasmic syncytium, derived from the central clear 

 protoplasm and continuous with the walls of the epiphysis rests 

 on the following evidence. The fibrils are first seen at the time 

 of the appearance of the central cavity, not after it is formed. 

 The cavity of the stalk is completed after that of the vesicle and 

 a syncytium of fibrils is seen as the central axis of protoplasm 

 disappears in the process of cavity formation. In staining reac- 

 tions and in structure the fibrils agree with the central proto- 

 plasmic mass, except at the periphery where they resemble, in 



Fig. 16. Pineal region in median section. Opsanus of 19 mm. H.E.C., series 

 no. 1188, section no. 203. X 84. 



these respects, the surrounding walls. There is, therefore, no 

 line of demarcation between the net -work and the protoplasm 

 of the walls. The latter have undergone no differentiation at 

 this time; there are as yet no cell boundaries. Bone has appeared 

 in the cranial roof and to its under side are fixed the connective 

 tissue bundles that radiate from the tip of the epiphysis. A com- 

 missural canal has replaced the transverse groove of the integu- 

 ment, seen in the 15 mm. larva, lying somewhat further in advance 

 of the end vesicle of the epiphysis than did the groove (fig. 16 C). 



