ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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a median and two lateral portions, which probably correspond to the 
portions which give rise further back to gut and notochord on the one 
hand, and to mesoblast plates on the other. 
The axial portion, beneath the thalamencephalon, degenerates ; the 
paired portions seem to form the foremost palingenetic pair of somites, 
or anterior head-cavities of Platt. What van Wijhe has reported as to 
the three foremost palingenetic head-somites in Scyllium and Pristiurus 
is confirmed by Hoffmann in regard to Acanthias. He promises a future 
discussion of the three posterior palingenetic and the four coenogenetic 
protovertebrae. 
Olfactory organ and nerves. Apart from the optic nerve, the olfactory 
diverges from all the other cranial nerves. As van Wijhe has shown, 
both the olfactory organ and its nerve arise from the anterior neuropore. 
Mouth and hypophysis. The hypophysis is formed very late, long 
after the oral opening. 
Development of Auditory Vesicle in Vertebrates.* — Dr. C. Poli 
has studied this in a variety of types. In Sauropsida, the first hint of 
the ear is a zone of thickened ectoderm alongside of the still open 
medullary groove. This zone proliferates and thickens, perhaps repre- 
senting a rudimentary somite. In relation to the segmentation of the 
medulla, the invagination occurs between the fourth and fifth neuro- 
meres. On the appearance of the gill-pouches the auditory zone extends 
ventrally by thickening of the branchial zone. The ventral and dorsal 
margins of the invagination fuse, and a vesicle is formed. The space 
occupied is at first the dorsal portion of the posterior half of the hyoid 
arch rudiment ; but when the vesicle is closed, the second branchial 
aperture corresponds to the boundary between the anterior two-thirds 
and the posterior third of the vesicle. The appearance of the auditory 
nerves is preceded by the development of a strand of spindle-shaped 
cells which forms the supporting tissue for the facial and auditory group. 
But these two nerves arise quite separately. The histogenesis of the 
auditory neuro-epithelium resembles in its earlier stages that of the me- 
dullary canal, supporting spongioblasts becoming distinct from the 
neuroblasts. 
In Selachians a thickened ectoderm-ridge appears in the earliest 
stages alongside of the medullary groove, and in this the auditory zone 
arises. Goronowitsch’s interpretation of the ganglion-ridgo in birds is 
corroborated in Selachians, — it gives off supporting elements to the 
peripheral nervous system. The first histogenetic changes in the audi- 
tory epithelium, which indicate the maculae acoustic®, begin earlier than 
in fowl-embryos. Two-thirds of the auditory vesicle lie in the posterior 
half of the hyoid arch region. After the vesicle has been formed there 
is still no appearance of lateral organs. 
In Anura there is no sensory-plate common to the three upper sense- 
orgaDs; what Goetto described as such is the ganglion-ridge. The 
invagination arises from an insinking of the lower layer of the ectoderm, 
and is thus from the first bounded in front by the superficial layer of 
the ectoderm. The vesicle is closed by a proliferation of the margins 
of the invagination. Before the vesicle is closed, the recessus labyrinth'i 
* Arch. f. Mikr. Anat., xlviii. (1897) pp. 6H-86 (2 pis.). 
