ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 497 



The genus Marionia was instituted by Vayssiere in 1877, and we 

 know of ten species. The account of the nervous system of M. quad- 

 rilatera shows that the central system is as small and flattened as in 

 Tritonia ; the cerebropleural ganglia are, especially on their lower 

 surface, very distinctly separated. In most of its characters it agrees 

 with what is found in the better known genus, and the same is true 

 of many of its other parts. 



M. affinis is a new species founded on a specimen found at Naples, 

 and apparently closely allied to M. tetraquetra ; its peculiarities are 

 pointed out, and there are some notes on M. tethydea and M. hlain- 

 villea, but the author does not seem to have arrived at any general 

 conclusions. 



Mollusc oida. 



Ova of Aseidians.* — A. Sabatier finds that, in Aseidians, the 

 ovary is first made up of an agglomeration of nuclei, derived from the 

 mesoderm, and united together by a small quantity of clear inter- 

 mediate substance. The ovary has the constitution and characters of 

 an embryonic connective tissue, in which " the protoplasmic atmo- 

 spheres " are not sharply distinguished ; and this structure is to be 

 found in such portions of the adult ovary in which there is a fresh 

 formation of ova. The egg arises from a corpuscle of this tissue, 

 which developes within itself one or more granules, which become 

 nucleolar, and which is itself the nucleus of the egg. Around the 

 nucleus there becomes arranged a layer of transparent colourless 

 protoplasm ; there then appears a very delicate amorphous capsular 

 egg-envelope, and below this there appear on the surface of the yolk 

 the follicular elements which become the follicular cells ; they are 

 small masses formed in the vitellus itself, and by it brought to its 

 surface. As they increase they form a continuous layer around the 

 egg. Below them and at their expense there is developed a second 

 membrane which lies on the yolk ; this subcapsular membrane may 

 become more or less thick. In certain cases the follicular cells 

 remain flattened, harden, and give rise to a thick and structureless 

 egg-envelope. The cells which, in the author's opinion, are impro- 

 perly called those of the testa arise from the yolk, of which they 

 represent an eliminated element ; they are imperfect cells and may 

 be spoken of as celluloid globules. The intra-vitelline corpuscles 

 are neither elements which have come in from without, nor capsular 

 cells which have made their way into the yolk, but are masses of 

 clear and finely granular protoplasm which are formed, by a process 

 of concentration, within the yolk itself ; finally making their way to 

 the surface, they are at first " capsular " and afterwards " granular " 

 cells. 



The author has investigated a large number of forms and he 

 points out the necessity of studying and comparing a number of 

 species if one would seriously attempt to come to any certain con- 

 clusions as to the history of ova which seem to be in any points 

 distinguished from those of other forms, and which exhibit many 



* Kev. Sci. Nat., xi. (1883) pp. 348-405 (4 pis.). 

 Ser. 2.— Vol. III. 2 K 



