506 SUMMARY OF CURRENT RESEARCHES RELATING TO 
and head-mesoderm. Among his results the following may be noted. 
Sense-organs and nervous system arise either exclusively or almost 
wholly from the inner layer of the ectoderm, the Nervenblatt of Goette. 
The ganglia of the cranial nerves arise from a ganglionic ridge, con 
tinuous at a certain stage with that of the trunk, and originally deriv- 
able from the medullary plate. But while the latter is composed in 
part from elements of the Deckscliicht and in part of elements of the 
Nervenschicht , the ganglionic ridge of the cerebral ganglia belongs 
wholly to the Nervenschicht. The ganglionic ridge gives origin to 
nervous tissue only. As to the head-mesoderm, it arises from an out- 
growth of the wall of the archenteron up to the level of the ecto- 
dermic hypophysial rudiment. No head-cavities were discerned ; the 
segmentation of the mesoderm is brought about by the formation of 
the gill-clefts. 
Regeneration of Intestinal Epithelium in the Toad during Trans- 
formation.*' — B. F. Kingsbury has studied this process, which resembles 
in many ways that which occurs in insects during metamorphosis. In 
the untransfoimed tadpole (of Bufo lentiginosus americanus), in which the 
legs are well developed, but before the arms have broken through, the 
intestine is lined by simple columnar epithelium. Connected with the 
epithelium and hanging down from it, or partially intercalated between 
the bases of the columnar cells, are clusters of three or four small cells 
with scanty protoplasm, which have been generally compared to the 
crypts of Lieberkiihn in mammals. After the arms have been put 
forth, these cell-clusters have increased markedly in size, and karyo- 
kinetic figures are abundant in them. From these cell-clusters the new 
epithelium of the transformed toad is formed. At first they are simply 
solid halls of cells ; soon, however, a lumen appears in the middle of 
the ball, and it becomes converted into a hollow sphere or flask whose 
wall is a single layer of columnar cells. The spheres grow, the side 
toward the old epithelium opens, the neighbouring crypts (spheres) 
mpet one another and fuse, and in that manner a new continuous epithe- 
lium is formed. The whole epithelium degenerates and disintegrates, 
and is pushed into the lumen of the intestine, where it remains as a 
mass of debris. If the cell-clusters in Amphibians correspond to the 
crypts of Lieberkiihn in Mammals, the above observations favour Bizzo- 
zero’s theory of the mode of regeneration of intestinal epithelium, 
namely, that the crypts of Lieberkiihn are the seats of the regenerative 
activity. 
Share of the Ectoderm in forming Mesenchyme.f — Herr H. Lund- 
borg has studied the development ot the cliondrocranium in dogfish, 
salmon, frog, Siredon , &c., and sides with those who conclude that the 
mesenchyme is helped in its work by formative elements. He main- 
tains that in the lower Yertebrata the mesenchyme may gain elements 
from any germinal layer (ecto-, endo-, or mesoderm), and he therefore 
doubts whether the doctrine of the definite Specificitdt of tissues, which 
Rabl and others uphold, can really be justified. 
* Trans. Amer. Micr. Foe., xx. (1899) pp. 45-8. 
t Morphol. Jahrb., xxvii. (1899) pp. 242-62 (2 pis. and 6 figs.). 
