Fig. 24.—Embryonic nerve-cells.
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]
142. With respect to the transition of the spindle-shaped cells into fibrils, since there is a gap in the observations of Boll, and since those of Schmidt are subsequent to the disappearance of the cells, and in both cases all trace of nucleus has disappeared, I suggest that we have here an analogy with what Weismann has recorded of the metamorphoses of insects. In the very remarkable memoir of that investigator[172] it is shown that the metamorphoses do not take place by a gradual modification of the existing organs and tissues, but by a resolution of these into their elements, and a reconstruction of their elements into tissues and organs. The muscles, nerves, tracheæ, and alimentary canal, undergo what may be called a fatty degeneration, and pass thence into a mere blastema. It is out of these ruins of the old tissues that the new tissues are reconstructed. On the fourth day the body of the pupa is filled with a fluid mass—a plasma composed of blood and dissolved tissues. The subsequent development is thus in all essential respects a repetition of that which originally took place in the ovum.[173]
Two points are especially noticeable: First, that in this resolved mass of granules and fat globules there quickly appear large globular masses which develop a fine membrane, and subsequently nuclei. A glance at the figure 51 of Weismann’s plates reveals the close resemblance to the earliest stages of nerve-cells; and the whole process recalls the regeneration of nerves and nerve-centres after their fatty degeneration.
Secondly, the nerves reappear in their proper places in the new muscles, and this at a time when the nerve-centres are still unformed; so that the whole peripheral system is completely rebuilt in absolute independence of the central system. The idea, therefore, that nerve-fibres are the products of ganglia must be relinquished. This idea is further discountenanced by Boll’s observations, which show that the fibre-cells are from the first different from the ganglionic cells; and by the observations of Foster and Balfour, that “fibres are present in the white substance on the third day of incubation”; whereas cell processes do not appear until the eighth day. Foster and Balfour are inclined to believe “that even on the seventh day it is not possible to trace any connection between the cells and fibres.” In the later stages, the connection is perhaps established.[174]