ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
33 
underlying tissue are merely due to the obliquity of the plane of section, 
and the consequent overlapping of the two layers ; the splitting off of 
the lower layer of the ectoderm appears to be due to resistance offered to 
the section-knife ; no actual inwandering of this layer into the mesen- 
chyme can be observed. 
Fins of Fishes.* — Prof. C. Gegenbaur maintains that there is funda- 
mental uniformity in the fin-skeleton of Selachii, Crossopterygii, and 
Dipnoi. The type' is a stem with biserial radials — an archipterygium. 
Whether the stem is much jointed or but little is immaterial ; even in 
the young Ceratodus it is not jointed (Semon) ; between recent Selachii 
with few joints and Xenacanthinse with many there are all possible 
stages ; similarly the biserial radials are plain enough in the old Xena- 
canthinse. In Selachii the distal modification of the hind-fin as a 
“ mixipterygium ” is associated with a loss of median radials ; the loss 
of the mixipterygium in Teleosteans, &c., is associated with great reduc- 
tion of the hind-fin. Gegenbaur gives this comparative table as to 
fore-fin : — 
Crossopterygii. 
Dipnoi. 
Selachii. 
Fin-stem 
^ "V 
long sbort radials 
probably heterogeneous 
Phanero- uniform 
pleurini 
Holoptychius Coelacan- Polypterini 
tliini 
Stem long, much-jointed, basal joint 
specially differentiated. 
Radials 
biserial uniserial absent 
Ceratodus Protopterus Lepidosiren 
Radials only biserial 
terminally. 
Stem 
/ " " > 
much with few joints ; 
jointed lateral radials 
reach shoulder 
Xenacan- girdle 
thinae 
Recent Selachii 
But it is perhaps more appropriate to notice that Gegenbaur adheres 
to his original theory in regard to the origin of paired fins. As is well 
known, he finds the material for fins in the branchial arches. He seeks 
to show why there is no longer any ontogenetic evidence of this, and 
maintains that an aetiological explanation which finds the cause of an 
origin in the final function is just “ die alte Teleologie.” He derives 
the skeleton of a fin from a branchial arch and its musculature from a 
portion of the associated myomere which, with new attachments, acquired 
a new function. 
Development of Olfactory Organ of Torpedo.f — Mr. John F. Holm 
describes as the first stage an ectodermal thickening a little to the side 
of the neuropore ; it is connected with the brain at one spot through 
rows of cells. Secondly, the organ begins to involute and increases in 
thickness, while the brain has now retracted. Connecting nerve-cells 
between the brain and the olfactory organ represent the beginning of 
the olfactory nerve or ganglion. It seems probable that “ during the 
process of separation of the brain from the organ some of the connecting 
cells elongate, send out fibres, and thus form a beginning of an olfactory 
nerve or ganglion, so that a kind of connection always exists from the 
very first.” Thirdly, the organ forms a pit or sack and the olfactory 
bulb is differentiated, giving off numerous nerve-fibres. 
* Morph. Jahrb., xxii. (1894) pp. 119-60 (5 figs.), 
t Anat. Anzeig., x. (1894) pp. 201-7 (6 figs.). 
D 
1895 
