ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
37 
Dentition of Wombat.* — Dr. C. Rose finds tliat Phascolomys Wombat 
in tlio foetal state lias a double set of teeth — a rudimentary milk-set of 
limited growth, and a permanent set with prismatic teeth. The teeth of 
the first set, to which perhaps there belongs a non -calcified premolar 
rudiment, are probably in part absorbed before they cut, or else they 
fall out in early youth. Of the incisors of the second series two must 
have been lost from the upper jaw, and one from the lower jaw. 
Ganglion Ridges in the Head of Birds.! — Herr N. Goronowitsch 
begins by noting that the so-called ganglion-ridges ( Ganglienleisten ) are 
most strongly developed on that part of the embryonic head on which 
no ganglia develope, namely, in the mid-brain region. Difficulties of 
this sort led him to reinvestigate the matter. As regards trout and pike 
he has convinced himself that the entire strongly developed portion of 
the ridges, in the thalamencephalic and mesencephalic regions, has 
nothing to do with the development of ganglia and nerves. It forms 
mesenchyme tissue, connective and probably skeletogenous. The same 
is true for birds. 
We cannot follow the author in his description of the state of affairs 
in twenty-seven stages in the development of the chick, but some general 
results may be noted. Goronowitsch distinguishes as primary ridges 
those which appear first in the region of the thalamenceplialon and 
mesencephalon ; secondary and tertiary ridges appear later in the region 
of the medulla oblongata. For the bird, as for Teleostei and Selachii, 
it is the case that the ectoderm shares in the formation of mesenchyme 
tissue, for this is the fate of the whole of the primary ridges. The 
medullary plate must be regarded as a composite Anlage , for it gives 
origin to things so different as the central nervous system and various 
structures of a connective-tissue nature. 
In the formation of the secondary ridges, the ectoderm and the 
medullary plate have an equal share. The secondary ridge in birds is 
never so strongly developed that it could produce the whole, apparently 
unified, mass of tissue which unites the dorsal arch ( Gewolbe ) of the 
medullary canal with the “branchial sense-organs.” The structure 
which has been interpreted as the rudiment of a nerve or ganglion 
passes without break into the axial mesoderm and fuses with the middle 
plate. The so-called “ branchial sense-organs ” of early stages represent 
the points of fusion of the most heterogeneous structures, and soon dis- 
appear. The connection of the arch of the medullary tube with the 
middle plates in the region of the second and third somites occurs 
through the formation of a tissue-strand, which, like the secondary 
ridges, is formed from two sources, the tissue of the ridge and the meso- 
derm arising from a proliferation of the middle plate. This strand the 
author calls the “ second periaxial strand.” The “ first periaxial strand,” 
which appeared at an earlier stage, established a similar connection be- 
tween the arch of the medullary tube and the middle plate. Goronowitsch 
follows the modifications of the periaxial strands and describes the 
incipient stages of the trigeminus, facialis, glossopharyngeus, and vagus. 
The appearance of the strands is contemporaneous with the 
* SB. Preuss. Akad. d. Wiss., 1893, pp. 749-55 (3 figs.), 
t Morphol. Jahrb., xx. (1893) pp. 187-259 (4 pis.). 
