140. The admirable investigations of Franz Boll have given these observations a new significance. He finds in the cerebral substance of the chick on the third or fourth day of incubation a well-marked separation between the neuroglia and nerve-tissue proper. Fig. 24, A, represents three nerve-cells, each with its nucleus and nucleolus, and each surrounded with its layer of neuroplasm. The other four masses he regards as nuclei of connective tissue. Three days later the distinction between the two is more marked (Fig. 24, B). Not only have the nerve-cells acquired an increase of neuroplasm, they also present indications of their future processes, which at the twelfth day are varicose (Fig. 24, C). (All this while the connective corpuscles remain unchanged.) Although Boll was unable to trace one of these processes into nerve-fibres, he has little doubt that they do ultimately become (unite with?) axis cylinders. [Illustration: Fig. 24.--_Embryonic nerve-cells._] [Illustration: Fig. 25.--_Embryonic nerve-fibres._] It is difficult to reconcile such observations with the hypothesis of the cells being simply points of _reunion of fibrils_. We see here multipolar cells before any fibrils appear. Respecting the development of the white substance, i. e. the nerve-fibres, Boll remarks that in the _corpus callosum_ of the chick the first differentiation resembles that of the gray substance. The polygonal and spindle-shaped cells represented in Fig. 25, A, are respectively starting-points of connective and neural tissues. The spindle-shaped cells elongate, and rapidly become bipolar. This is supposed to result in the whole cell becoming transformed into a fibre, the nucleus and nucleolus vanishing; but the transformation is so rapid that he confesses that he was unable to trace its stages; all that can positively be asserted is that one or two days after the appearance presented in Fig. 25, B, the aspect changes to that of fibrils. The columns of polygonal cells between which run these fibrils, he regards as the connective corpuscles described by several anatomists in the white substance both of brain and cord, and which are sometimes declared to be multipolar nerve-cells.[170] 141. Dr. Schmidt’s observations on the human embryo were of course on tissue at a very much later stage. According to him, the fibrils of the axis cylinders are formed by the linear disposition and consolidation of elementary granules. The fibrils thus formed are separated by interfibrillar granules which in time become fibrils. Not earlier than three months and a half does the formation of individual axis cylinders begin by the aggregation of these fibrils into minute bundles, which are subsequently surrounded by a delicate sheath.[171]