266 The Development of the Neuroglia 



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EXPLANATION OF FIGUEES ON PLATES I-V. 



All the figures are from transverse sections of the spinal cord, and with 

 the exception of Fig. 1, which is from the rabbit, all illustrate conditions 

 found In the pig (cervical region). The oil immersion (Zeiss) was used in 

 all the drawings. 



PLATE I. 



Fig. 1. From the neural tube of an embryo rabbit shortly before the 

 closure of the tube. Taken from His, 89. Showing a stage when the wall 

 is composed of distinctly outlined, individual cells, a = inner zone; (/=: mi- 

 totic nucleus or germinal cell; m = peripheral zone, position of the later 

 mantle layer, x 920. 



Fig. 2. Pig of 5 millimeters, unflexed. Just after the closure of the neural 

 tube. Segment from lateral wall of tube showing the disappearance of cell 

 membranes and the beginning of the consequent fusion of the cell proto- 

 plasm to form a syncytium, a = inner zone; f/i^ mitotic nuclei or germinal 

 cells; m = beginning of mantle layer; mZt = internal limiting membrane; 

 »• = radial columns of protoplasm, x 920. 



Fig. 3. Pig of 7 millimeters, unflexed. Segment from ventro-lateral wall 

 of neural tube. Showing the earliest form of the syncytium and the second 



