Irving Hardesty 267 



stage in the mantle layer. ?»a dinner zone or ventricular border of the 

 ependymal layer; fir=: nuclei in mitosis or germinal cells; mZi =: membrana 

 limitans interna; mZe = membrana limitans externa; r = radial, axial fila- 

 ments of the syncytial protoplasm; p^anlage of pia mater, x 920. 



Fig. 4. Pig of 10 viillimeters, " crown-rump measurement." Segment from 

 lateral wall of neural tube showing more pronounced radial arrangement of 

 the syncytium, the final disappearance of all cell membranes, and a third stage 

 in the structure of the mantle layer, a =z inner zone or ventricular border 

 ot ependymal layer; b^r^ boundary between nucleated layer and mantle layer; 

 £/ = mitotic nucleus; m^ mantle layer; viH and mZe = internal and external 

 limiting membranes; rr= axial filaments; pr=anlage of pia mater. By the 

 author's mistake in measurement the original drawing is too much reduced 

 in this figure. To conform to the scale of the preceding figures its long axis 

 should be about i/^ inch greater. 



PLATE II. 



Fig. 5. Pig of D millimeters, flexure beginning. Showing the stage of the 

 first ingrowth of blood-vessels, the abundance and locality of dividing nuclei, 

 the migration of the nuclei, the beginning of the middle nucleated layer, and 

 the general appearance of the syncytium at this stage. A portion of the 

 surrounding connective-tissue syncytium is included in the drawing. The 

 depression along the ventral aspect of the specimen is greater than is usual 

 at this stage (see Fig. 6). 6c = blood capillary; cs = connective-tissue syncy- 

 tium; (Z — dorsal aspect of tube; ep = ependymal layer; mw = middle nucle- 

 ated layer; mv = mid-ventral portion of mantle layer; v7i=zanlage of ventral 

 horn; sgr=: spinal ganglion; other reference letters = same as in Fig. 4 

 X 170. 



Fig. 6. Photograph of section from pig of 15 millimeters, "crown-rump 

 measurement." Showing the low-power appearance, relative thickness and 

 extent of the mantle layer, the general appearance of the syncytium, the 

 appearance of the middle nucleated layer {vm) resulting from the further 

 migration of nuclei from the ependymal layer (ep). Fig. 7 is taken from 

 the ventral portion of sections similar to this. d = dorsal portion of epen- 

 dymal layer about which the mantle layer does not extend; (f Ti = anlage of 

 dorsal horn; mv = mid-ventral portion of mantle layer; other reference letters 

 =r same as in Fig. 4. x 90. 



Fig. 7. Combination drawing from sections of pig of 15 millimeters. The 

 lower part of the drawing is taken from the ventral portion of the section 

 shown in Fig. 6; the upper part is from a section of the same stage but 

 stained by the silver method. The drawing allows a comparison of the results 

 obtained by the two methods and shows in more detail the appearance of the 

 syncytium in the nucleated layer {mn) and the mantle layer (m), and the 

 greater complexity of the protoplasmic filaments along the border line between 

 the two (&). &v = blood-vessel; cs = connective-tissue syncytium; ?)iv =r dif- 

 ferently . arranged mid-ventral portion of the mantle layer; r^^r radial fila- 

 ments of syncytium as shown by the silver method; other letters same as in 

 Fig. 4. X 320. 



Fig. 8. Photograph of transverse sections from pig of 30 millimeters. 

 Showing further advancement of the changes begun in Fig. 6, including the 



