NOTES AND MEMORANDA. 
267 
The spots commenced to show themselves, but the tissue withered on 
all the leaves: the parasites only developed in a very incomplete 
manner, and did not appear the following year. The fungus is there- 
fore entirely localized on the foliar and deciduous organs ; it is fully 
developed only on the young organs. 
It appears then that it would be sufficient to produce the dis- 
appearance of the Bhytisma to destroy all the spotted leaves which 
fall in the autumn, but this presupposes that the small corpuscles 
(spermatia of Tulasne), produced in enormous numbers on the living 
leaves (Xyloma), are not able also to produce the parasite. 
The red spots of the plum tree, produced by the Poly stigma ruhrum, 
an ascomycetous fungus of quite another group, and much more 
dangerous, have probably a very analogous history.^ 
The Beproduction of Hydra (Note by M. Korotnefif, communicated 
by M. de Lacaze Duthiers to the French Academy). — “ In spite of 
its abundance, the fresh-water Hydra presents a great number of 
peculiarities insufficiently studied, and particularly the reproduction 
of the sexual elements, and the embryonic development of the in- 
dividual itself. 
These phenomena have been deseribed in a fairly detailed manner 
by Kleinenberg, in his ‘ Monograph of Hydra.’ According to his 
researches, the cells are found below the ectodermal elements 
(interstitielles Gewehe), and form an agglomeration whieh serves to 
reproduce the ova as well as the spermatozoids. The development of 
the ovum is aeeomplished as follows : one of the cells of the agglo- 
meration inereases remarkably, and swallows up the surrounding cells 
— in other words, it feeds itself upon them. The nucleus is changed 
into a germinal vesicle, and at last, the cell itself represents the ovum 
of the Hydra, which is thus, according to its origin, a monocellular 
and ectodermic formation. 
The granulations of a definitively formed ovum serve to produce 
the larger elements, which Kleinenberg describes under the name of 
pseudo-cells {Pseudocellen, Kh), 
After a detailed description of the segmentation, the German 
savant passes to the formation of the blastoderm, as an immediately 
sueceeding phenomenon to segmentation. The blastoderm consists of 
a layer of cells, forming by itself the entire envelope of the ovum. 
Kleinenberg eonsiders the blastoderm to be an embryonic epithelium, 
not taking part in the ulterior formation of the Hydra, and thrown 
off as an envelope at a certain period of the development ; for this 
reason the adult Hydra is an animal destitute of epithelium. 
My own researches, made upon Hydra fusca, completely contra- 
diet those of Kleinenberg. Nevertheless, conformably with his 
researches, I have seen an agglomeration of cells, of ectodermic 
elements, whieh I consider simply embryonic cells, which serve to 
rcproduee different ectodermic elements. One of these cells grows, 
and its nucleus changes into a germinal vesicle. At the same time, 
the peripheral elements of the agglomeration separate, forming a row 
* M. Max Cornu, in ‘ Comptes Kendus,’ vol. Ixxxvii. p, 178. 
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