Talk:Paper - The histogenesis of the cerebellum (1895)

From Embryology

By C. L. Herrick.

We are impelled by a recent paper from Dr. Schaper to review the utterances of this journal respecting the cerebellum. In the article referred to' several of these publications are referred to but are dismissed with the remark, which is made to apply to all previous work, " Ihre Resultate jedoch erheben sich kaum iiber Vermuthungen." Welcoming as we do the circumstantial confimation of our own investigation we can but feel that the positive results we had published and which rested on long and often-verified study merit a recognition they have not received from Schaper or other subsequent writers. Reviewing Schaper's work we find nothing new respecting the morphology of the organ until we reach the statement that the origin of its substance is not a thickening of the dorsal wall but the migration of elements from two lateral analags. But His has already shown that the cerebellum arises from the Fliigel" platten or lateral aspects, and the writer in 1891 gave a detailed account of the process in the rodents and has repeatedly verified the process in other mammals and lower groups. Plate I of Vol. I of this journal illustrates the morphological relations so fully that it has never seemed necessary to amplify the description. Pages 11-13 of this same volume give a description of the way in which the dorson of the cerebellum (which is originally devoid of cells) receives contributions from the everted walls of the lateral recesses and the caudal margin. Of course it is not understood that such folds are mechanically produced but that the sequences of proliferation follow so as to accomplish a result which can only be thus imaged. Em

'A. Schaper. Die Morphologische und histologische Entwicklung des Kleinhirns der Teleostier. 1894.

Herrick, Histogenesis of the Cerebellum. 67

bryological literature is full of such usages and no confusion was anticipated therefrom, though it has resulted in the case of some reviewers untrained in embryology. On p. 669 of Schaper's paper we read that " in der Deckplatte der Medianfurche und in den I5egrenzungsgebieten der beiden Ressus laterales wir drei Orte vor uns haben, von denen nunmehr das neue Material zur weiteren Entwicklung das Kleinhirns bezogen wird." These statements are anticipated in the first number of this Journal and fully illustrated, Schaper then proceeds to discuss the superficial cell layer of the cerebellum which, he says, " findet sich, so veil wir wissen, nur im Kleinhirn." He indeed says " I believe that my observations derived from faultless preparations warrant me in supporting Herrick's theory and to extend it by adding that, in fishes at least, the ' Deckplatte der Medianfurclie ' participates in this process, a fact which seems to acquire special significance from considerations to be mentioned later." The descriptions and figures given in the article referred to and subsequently will show that this locus was especially recognized by us as a source of proliferation. Thus on p. 13, "While in this case [black snake] the organ does not undergo reflection but remains a leaf-like organ, longitudinal sections indicate that proliferation is most rapid near the tip, and a dense clustre of cells is pushed dorsad and cephalad upon the dorsal surface." Respecting the source of the elements the same passage continues "There is every reason to conclude from these sections that the cells of Purkinje likewise spring from near the ventricular border. At the tip and lateral margins, the layer of these in the embryo comes in contact with the epithelium, and the cells which are obviously multiplying rapidly, either spring from the epithelium by subdivision of its undifferentiated cells or the multiplication of special germination cells. The neuroblasts at first become spindle-shaped and give rise to a process which passes cephalad. Nearer the base the layer of Purkinje cells is represented by a thick nucleary zone from the epithelium. At the very base, however, there is a superficial dorsal cell-aggregate which likewise seems to have its origin in the epithelium of the ventricle," On the ninth page

68 Journal of Comparative Neurology.

the origin of the superficial layer is described. "This remarkable assemblage of superficial cells with dividing nuclei, karyokinetic figures and other evidences of proliferation may be readily seen in the cerebellum of old embryos of the guinea-pig.

We observe that in an older embryo a much larger portion of the dorsal surface has been thus covered, and evidently from behind forward. A transverse section of the cerebellum near its base in a mouse embryo of a little older stage, shows that the entire surface has now been covered by the proliferating zone, but that it is curiously double, with a layer of white fibres separating the two zones. It is also observed that the walls of the nerve tube at the recessus lateralis of the fourth ventricle are very thin, and consist of rapidly proliferating and hence closely-packed cells which pass from the ental to the ectal surface. This section suggests that possibly the lateral and caudal portions of the ventricular surface of the cerebellum may be the sources of the proliferating superficial layer of the dorson." The form of this statement m.ay have led Schaper to speak of our statements as '* Vermuthungen " but the article goes on to afford proof that this " suggestion is valid " by a study of the origin of the recessus lateralis. On p. 12 we have "From either side there extends a curious upward fold containing a cavity. The source of the growth is evidently the walls of the cavity as witness the numerous cells clustred at that point." " It will be noticed also that the entire dorsal portion of the organ is affected by this overlapping growth from behind and the sides, so that a groove is formed on either side of the meson, dorsally. " "Fig. 7 well exhibits the extent to which the gray matter of the dorso-lateral aspect of the medulla is derived from the epithelium of the recessus lateralis." One word also respecting the idea that the cerebellum is peculiar in the possession of a transitory superficial proliferating zone. Fig. 4 of Plate II of Vol. I of this Journal was drawn to call attention to the development of a proliferating zone in the cerebrum. Though this zone is not strictly superficial it nevertheless corresponds to the layer in question and is a centre of migration. Similar stages have been observed in other parts of the brain, as in the optic lobes,

A more detailed account of the fate of the superficial layer is given in the supplement to Wood's Reference Handbook of the Medical Sciences. From the article on Histogenesis of the Nervous System, (p. 692) we quote the following paragraph : "The ectal portions of the massive organs, especially the cerebrum and cerebellum, become somewhat rapidly clothed with a dense mass of granules which subsequently disappear (Fig. 496). Professor His suggests that these are white blood-corpucles which, after wandering from the sinuses surrounding the organ, make a temporary halt near the surface before passing throughout the substance of both gray and white matter. The period at which they appear in the human embryo is about the end of the second month. While not doubting the existence of amoeboid cells, as above described, the writer has suggested other sources for part of these cells. It will be noted that the peripheral zone of cells arises only after direct communication with the ventricles has been cut off by interpolated white matter. Subsequently the number of granules {e. g., beneath the Purkinje layer in the cerebellum) rapidly increases. Their source can only be the peripheral collection of granules. Fig. 496 shows that, at this stage, there is often rapid multiplication going on within that layer. Careful study has not revealed any other source for these proliferating granules than diverticles of the epithelium, from which migration in this more round-about way is kept up. It is a familiar fact that the earlier granules are derived from the proliferating neuroblasts of the ventricular surface : it is inherently improbable that the subsequent ones should spring from as distinct a source as the pia or its vessels. The peripheral proliferating layer soon disappears by the migration of its elements toward the ventricle."

One statement made by Schaper (p. 693) is in our belief inapplicable to fishes or at least is not universal. He says, " Das urspriingliche 'embryonale stutzgeriist ' geht bis auf die zu eigentlichen ependymzellen sich wandelnden elemente mit aller warscheinHchkeit friizeitig zu Grunde. " We have shown that in fishes and reptiles the original spongioblastic skeleton persists and may be easily made out in properly prepared sections of the optic tectum and other regions extending from ectal to ental surfaces. Moreover the much-mooted " gelatinous tracts " are frequently but the elongated fibre-like connections between the two free surfaces which have been separated by the distortions due to growth. Compare Plate XXVIII, Figs, i and 7 in Festschrift f. Leuckart, and discussion on p. 45-46 of this Journal for May, 1892. In an article in the Denison Quarterly, republished in this journal for Dec. , 1892, pp. 141-142, the origin of the cerebellar elements from proliferations of epithelial origin is again described. On p. 86 of the June number for 1893, is a discussion of the primary or transitory layer in the cerebrum. See also Anatomischer Anzeiger, VII, 13, 14. Dr. Schaper deserves great praise for his thorough study of the subject and it in no way detracts from the merit of his work to call attention to the fact that he does not stand alone in his positions.