ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
115 
In the remaining mass continued cell-multiplication goes on. A 
stratum of white matter appears dividing an upper part — the optic 
lamina — from a larger lower part — the cerebral ganglion. From the 
optic lamina there differentiates in the solitary form the horseshoe- 
shaped unpaired eye ; in the aggregate form there arise the optic spheres 
which give origin to the rudimentary posterior eye and the two com- 
pletely developed anterior eyes. 
Grey and white matter are differentiated in the brain. From a zone 
of large nerve-cells motor nerves arise ; very small cells, present in great 
abundance, are probably sensory. From the anterior surface of the 
ganglion there arise two mixed nerves with two roots — an inferior in the 
large motor cells, a superior in the small sensory cells. The nerves 
supply the buccal muscles and the sensitive epithelium of the buccal 
cavity. 
Tunicata of Norwegian North- Atlantic Expedition.* — The hand- 
somely illustrated volume which deals with these forms consists of five 
parts. The Synascidiae are discussed by Mr. H. Hvitfeldt-Kaas, the 
Ascidise Simplices and Composite by K. Bonniere, who deals also with 
the budding of Distaplia magnilarva and Pyrosoma elegans. Mr. J. Kiser 
gives a list of Norwegian simple Ascidians ; while Mr. J. Hjort has an 
essay entitled “ Germ-layer Studies based upon the Development of 
Ascidians.” 
Of the 24 species of Synascidians, a third are new to science, but the 
slight amount of literature on the subject has made the determination of 
the forms a matter of some difficulty. 
Eleven species of Simple and ten of Compound Ascidians were col- 
lected by the Expedition, and most were represented by a single speci- 
men, not always in a good state of preservation. The development of 
the buds of Pyrosoma was found to proceed according to the same laws 
as those which govern gemmation in the Synascidiaa ; the outer vesicle 
has no share in the development of the bud, the most important organs 
being formed from the inner vesicle. The peribranchial cavities and the 
nervous system are not formed, as in the Synascidim, by a simple evagina- 
tion of the wall of the inner vesicle, but this is due to the great thickness 
of the layer of cells. The difference between the present results and 
those of Seeliger may be well explained by the difficulties of the inves- 
tigation of Pyrosoma. 
Our space does not permit us to give a full account of Mr. Hjort’s 
memoir ; we must content ourselves with saying that, after describing 
the embryonic development of Ascidians, the bud-rudiment in the various 
groups, and the formation of the organs in the bud, he compares larval 
and bud development and discusses gemmation in Ascidians and the 
germ-layer theory, as well as Ascidian development and the biogenetic 
fundamental law. 
Tunicata of the ‘ Caudan ’ Expedition. — The few Tunicata col- 
lected by Prof. Koehler in the Bay of Biscay are divided between Prof. 
L. Roule, who takes the simple forms,! and M. M. Caullery,* who deals 
* DenNorske Nordhavs-Expedition, xxiii. (1S96). I. 27 pp. and 2 pis. ; II. 16 pp. 
and 2 pis. ; III. 23 pp. and 1 pi. ; IY. 15 pp. and 3 pis. ; Y. 72 pp. and 4 pis. 
t Resultats Scient. de la Campagne du ‘ Caudan,’ fasc. ii. (1896) pp. 355-8. 
x Op. cit., pp. 359 and 60. 
