682 _ REVIEWS AND BOOK NOTICES. 
If the endoderm and ectoderm are homologous in all the Metazoa 
[i.e. all animals except Protozoa] we then “have a right to suppose 
that these two cellular layers have in all the same histological 
concerns the central nervous system, which is developed in all 
animals from the ectoderm. 
‘ RRR EA it makes no difference if we should wish to know 
the origin of an organ, whether we seek for it in one or another 
animal kingdom, and receive a general signification. 
However, of all the types of organization, that which serves 
with their embryonic characters during their entire life; all the or- 
gans of the zoophytes are only a dependence a ah or the other 
of these layers, sometimes of the two layers un 
The polype form may be traced back with the ah eg ara 
to the Gastrula, all the parts of which are preserved without un- 
dergoing any great modifications during all the course of exist- 
ence. 
Conclusions. In the Hydractinie 1. The eggs are developed 
exclusively from the epithelial cellules of the endoderm. They 
remain, up to the time of their maturity, surrounded by the ele- 
m. 
2. The testicles and spermatozoa are developed from the ecto- 
derm; this organ results from the progressive transformation of 
a primitive cellular fold formed by invagination. 
3. There exists in the female sporosacs a rudiment of th 
ticular organ ; in the male sporosacs a rudiment of an oy Ary 
sporosacs are then sbigeaaque engaataied hermaphrodites. * 
Fecundation. consists in the u of an egg, a product A the 
e tes- 
Pg: 
* 
froin which the ectoderm are formed are Kaede a separated from 
those which are to form the internal layer of the embryo. Be 
The new individuality j is realized at the instant when the uni 
between the elements of opposed polarity has taken place, as 
solutely as a molecule of water is formed by the union of atoms 
of hydrogen and oxygen 
In a late paper on the embryology of jelly fishes Metz 
criticised the universal application of Haeckel’s Gastrea theory» 
nikoff has — 
