303 



6) Paladino, Ct., Contribuzione alia conoscenza sulla struttura e funzione 

 della vesicola ombelicale nell'uomo e nei mammiferi. Arch. Ital. 

 Ginecol. Napoli, Vol. 8, 1901, p. 127—134. 



7) Robinson, A. B,., The Nutritive Importance of the Yolk Sac. Jour. 

 Anat. and Phys., Vol. 26, 1892, p. 308—323. 



8) Saxek, f., Ueber die Entwickelung und den Bau der normalen 

 Lymphdrüsen und die Entstehung der roten und weißen Blut- 

 körperchen. Anat. Hefte, Bd. 6, 1896, p. 349—523. 



9) Selexka, E., Studien über Entwickelungsgeschichte der Tiere. Das 

 Opossum, Wiesbaden 1886; Menschen-Affen, Wiesbaden 1891 und 

 1899. 



10) Speb, Graf, Zur Demonstration über die Entwickelung der Drüsen 

 des menschlichen Dottersackes. Anat. Anz., Bd. 12, 1896, p. 76 — 79. 



Nachdruck verboten. 



The Migration of Medullary Cells into the Ventral Nerre-roots 



of Pig Embryos. 



By E. W. Carpenter and R. C. Main (Urbana, 111., U.S.A.). 

 With one Figure. 



In studying the development of spinal nerves in pig embryos, the 

 writers have been interested in the evidence for the migration of 

 medullary cells into the ventral roots. At the time when the first 

 motor fibers grow out of the embryonic spinal cord, breaches of con- 

 siderable extent appear in the external limiting membrane. Through 

 these apertures the nerve-fibers pass out into the mesenchyme to form 

 the ventral nerve-roots. In pig embryos 11 mm. long we have seen, 

 in many sections passing through the roots of spinal nerves, what 

 appear to be medullary cells accompanying these ventral fibers. The 

 cells have been found just inside the external limiting membrane, in 

 an intermediate position half in and half out the neural tube, and in 

 the base of the nerve-root just outside the limiting membrane. A few 

 sections show continuous lines of medullary cells, touching end to end, 

 and reaching from the motor nidulus, across the boundary of the tube, 

 into the proximal part of the nerve-root. In such sections the cells 

 at the inner end of one of these lines frequently appear circular in 

 outline, those at the outer end appear elliptical, while in intermediate 

 positions gradations between these two shapes occur. This condition 

 possibly indicates that in passing out of the neural tube the form of 

 the medullary cells may change from approximately spherical to spindle- 



