ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 227 



lower half; it is only a useful mark, and has no morphological 

 significance. The cords are formed of an inner nuclear, and an 

 outer cortical layer ; the peripheral nerve-fibres arise either separately 

 or in bundles. This part of the subject is entered into with great 

 detail, and is illustrated by diagrams. 



5. The cerebral ganglia : in Fissurella these have undergone, during 

 evolution, a concentration, which has not, however, affected the 

 texture of the cerebral ganglia throughout the Ehipidoglossata 

 generally. The large pyriform and triangular cells which were seen 

 in the pedal cords, and especially in their pleurocerebral portion, are 

 here absent, and the largest cells are not larger than those of medium 

 size in the pedal cords. 



Speaking generally it may be said that this paper demonstrates 

 that the central nervous system and the peripheral ganglia of the 

 Ehipidoglossata consist of marginal ganglion-cells and of a central 

 plexus, which is formed from the processes of the ganglion-cells, and 

 in which the neui'ilemma takes no part. The peripheral nerves arise 

 from the cells and from the nerve-plexus. What changes may obtain in 

 more complicated nervous systems, which are phylogenetically younger 

 and in which there is great concentration and consequent elongation 

 of the trunks, does not seem to really oppose the view that the nerves 

 have everywhere a double mode of origin. Where the ganglia are 

 more compressed a process seems to have been going on by which the 

 larger cells have become grouped in outer cell-layers ; their continua- 

 tions have been pushed downwards, and they have anastomosed partly 

 with one another, and partly with smaller cells. 



Constitutioii of the Egg and its Envelopes in the CMtonidae.* — 

 Prof. A. Sabatier disagrees with the views put forward by Dr. Ihering 

 in 1878, in which the shell was regarded as being formed by the 

 structureless membrane which the earlier writer called the follicular 

 membrane, believing rather that the structureless membrane belongs 

 to the walls of the ovary and that the nuclei which Ihering regarded 

 as part of it are merely nuclei formed directly by the protoj>lasm 

 of the ovule, and eliminated peripherally. The author's observations 

 confirm the ideas of Fol, Eoule, Balbiani, and himself as to the 

 general origin of the cellular elements which form a follicle for the 

 egg ; and they seem also to confirm his special views as to the intra- 

 vitelline genesis of these elements, which he regards as being formed 

 by a kind of crystallization or condensation in the protoplasm of the 

 egg, and not as arising by direct expulsion from the contents of the 

 germinal vesicle. 



In the egg of Chiton polii, Sabatier observed a large number of 

 germinal vesicles, each of which had a single large refractive nucleolus, 

 which was homogeneous and very strongly coloured ; it was more or 

 less excentric in position, and the central part of the vesicle was 

 occupied by a more or less voluminous aggregation of chromatin- 

 grains which belonged to the nuclear plexus. A more or less large 

 number of rays were given off from this mass, and ended in the 



* Rev. Sci. Nat., iv. (1885) pp. 429-44 (2 pis.). 



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