I'LA'I'I'; c. 



20 'I'hi'ough tlu' ceiitcT of tlu' ubnoniial sinus tcnniiuilis, showiiifj; llu' l;ist. 

 pair of sensory roots (not last sensory root) in section. Observe especially the 

 large al)norinal sinus together with the rich vascular supply for the tissue immedi- 

 ately surrounding it. Note also the dorsal and ventral swelling caused by this 

 sinus and compare with photograph of the model (figs. 7 and 9) . They show every 

 evidence of being expanded by the cerebro-spinal fluid. From this point caudad 

 no nervous structures have developed in the spinal cord. .Vlso from this point 

 caudad the notochord gradually decreases in caliber. X 70. 



21 to 23 represent three transverse sections taken through the extreme pos- 

 terior, non-nervous, end of the spinal cord, which is composed solely of support- 

 ing tissue and undifferentiated embryonic cells. Figure 21 is the most cephalic, 

 and passes through the spinal cord a short distance behind the abnormal sinus 

 terminalis (figs. 9 and 20, S.T.). The spinal cord is still flattened here, but not 

 indented ventrad, and contains a normal central canal. In figure 22 there has 

 occurred a marked reduction in the size of the notochord. Observe that the 

 spinal cord has not become flattened as it has more anteriorly, where the noto- 

 chord is massive. With figures 21 and 22 compare figures 59 and 60, which are 

 taken from a similar region of the spinal cord from a 70 mm. Polistotrema embryo. 

 Figure 23 is the last of this series of drawings. It passes through the extreme 

 posterior end of the spinal cord, some 45 microns caudad of the last trace of the 

 notochord. Note the presence of a central canal, and that the cord has separated 

 into several processes, which further on become lost in the surrounding con- 

 nective tissue. X 70. 



PLATE 7 



EXPLANATION OF FIGURES 



24 to 31 From transverse frontal sections through the medulla and spinal 

 cord of several human embryos, drawn with the aid of an Edinger-Leitz draw- 

 ing apparatus, and reduced 3 diameters in reproduction. 



24 and 25 From transverse sections through the rhombic brain (frontal 

 through the embryo) of a 23 mm. human embryo; figure 25 passes through the V 

 root and posterior end of the cerebellar rudiments (lateral lobes), while figure 24 

 is from a more anterior section. The roof expansion (chorioid plexus) is shown 

 as a conspicuous black line in these figures. Everywhere within the boundaries 

 of the roof expansion, the cavity is filled not only with coagulated cerebro-spinal 

 fluid, but with embryonic red corpuscles. Whether these entered through veno- 

 lymphatic openings (C) or are the result of extravasations was not determined. 

 Wherever mesenchyme borders the roof expansion it is very vascular. It is 

 apparent that the roof expansion is under moderate internal pressure. At first 

 glance the roof expansion will show resemblance to the so-called first roof plate 

 expansion of the spinal cord of the 20 cm. Polistotrema, already figured, but its 

 later mode of development was shown to be very different. X 10. 



{Continued on -page 56) 



54 



