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POPULAR SCIENCE REVIEW. 
ZOOLOGY. 
Origin of the Ova in the Hy droids. — The first idea of what is commonly 
known as ‘ Alternation of Generations/ was suggested by the phenomena 
presented by certain hydroid polypes, in which the sexual functions 
seemed to be performed by special zooids, which, indeed, were often cast 
free in the medusoid form, so that the distinction into sexual and agamic 
zooids appeared in these cases particularly clear. Some recent observations 
indicate that we may have to modify our views as to the phenomena pre- 
sented by these animals. According to M. Goette, the ova of the Hydrella 
attain their complete development in the stem of the polype ; and according 
to M. Weismann, the several elements are developed in the stem and after- 
wards passed into the gonophores in Plumularia echinulata ; and the ova 
follow the same course in species of the genus Eudendrium. M. A. de Yarenne 
( Comptes JRendus, 16th August, 1881) adds some further particulars bearing in 
the same direction. He finds that in Campanularia jiexuosa the ova occur 
in the endoderm of the stem before any gonophores have made their appear- 
ance, and that they are simply endodermic cells differentiated, the cells 
being found in all stages of modification. The gonophore is at first a mere 
diverticulum of the wall of the polype, into which both ectoderm and endo- 
derm pass, carrying with them the cells which have been transformed into 
ova. The same phenomena were observed in a variety of Plumularia 
echinulata met with at Roscoff, and in Sertularia pumila. Thus in these 
three species in which the sexual generation is represented by permanently 
attached gonophores, the ova are not developed in the gonophore but in the 
stem of the supposed agamic generations. 
In Gonothyrcea Loveni the medusiform gonophores do not become 
detached from the polype ; the ova also originate from the cells of the endo- 
derm within the stem, and are carried with the neighbouring tissues into 
the blastostyle and gonophore, completing their evolution in the undetached 
semi-medusae. 
In the Tubularian Podocoryne carnea and the Campanularian Ohelia 
geniculata f which give origin to free-medusae, M. de Varenne observed a 
similar sequence of phenomena. He found that in the region from which 
the medusae are to bud, ova are produced by the modification of the cells 
of the endoderm. When the medusae begin to bud they are at first simple 
diverticula of the two layers forming the wall of the polype, and the ova 
are carried with them into young buds ; they are at first very small, but 
are rapidly developed, and attain their maturity after the medusa has been 
cast off. M. de Varenne sums up his results as follows : — 
'We are thus led to conclude, — 1. That in the species above cited, in 
which the sexual generation is represented by gonophores persistently 
attached to the hydroid polype, or by semi-medusae, or free medusae, the 
ova originate in the interior of the polype itself, and not in these gonophores 
or medusae. 
‘ 2. That the ova are only differentiated cells of the endoderm, and that 
in them may be observed all the stages of transition between an ordinary 
endodermic cell and a fully developed ovum. 
‘ 3. That the ova are conveyed into a bud, which is at first only a diver- 
