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SUMMARY OF CURRENT RESEARCHES RELATING TO 
of the dorsal ectoderm. The dorsal plate, when formed, gives rise to 
the typical organs of the Yertebrata. Attention is particularly called 
to the fact that in all Vertebrates the notochord and the axial mesoderm 
primitively formed a continuous Anlage , which only secondarily breaks 
up into the median notochord and the lateral mesodermal plates. The 
author thinks that this result is of great importance for the comprehension 
of the mode of origin of the notochord and of the axial mesoderm. In 
comparing the mode of formation of the mesoderm in Vertebrates, one 
finds that there are only two points of constant occurrence. The first 
is that, as already said, the notochord and the axial mesoderm arise from 
a continuous Anlage , and the second is that the mesoderm is always a 
continuous structure which owes its origin to the elements of the two 
primary germinal layers. This leads the author to deny the homology 
of the mesoderm as a germinal layer through the whole series of Verte- 
brata ; for example, the mesoderm of Amphioxus cannot be homologous 
with the mesoderm of Amphibians, for in the former it is axial only, and 
not peripheral. 
Phylogenesis of the Amnion.* — Prof. A. A. W. Hubrecht has 
published a general essay on the phylogenesis of the amnion and the 
meaning of the trophoblast. Kejecting all previous explanations of the 
phylogenetic origin of the amnion as being unsatisfactory, he suggests 
that it is very probable that the Protamniota were viviparous Amphibia 
in which the pseudomeroblastic germinal vesicle arose by a collection of 
fluid between the cells of the hypoblast. If this be so the germinal 
vesicle of Monodelphous Mammals corresponds to a stage which is 
phylogenetically older than that of the Sauropsida. He suggests that 
the amnion in this primitive stage began to be formed from the covering 
layer of the Amphibian epiblast. In the further development of arrange- 
ments to allow of osmotic exchange between the vessels of the embryo 
and the mother, a closed trophoblast arose ; this invested both the 
embryo and the amniotic vesicle. This was effected in such a way as to 
give opportunity for the allantois to develope in various directions ; on 
this hypothesis the trophoblast is homologous to the covering layer of 
the Amphibian epiblast. The actual separation of the internal lining 
of the amnion from a trophoblast layer several cells thick occurs at the 
present day in the Hedgehog, and a similar but shortened process obtains 
in P ter opus and Cavia. In the mouse and some other rodents the 
amnion arises by the formation of folds in epiblastic cell layers, and 
here both the embryo and the amnion lie within the trophoblast. 
Several series are described, and it is pointed out that a sharp separation 
between the embryonic shield and the trophoblast is the primitive con- 
dition. Both amnion and trophoblast are therefore closely allied in 
their phylogenesis, and both are to be regarded as products formed by 
separation from the outer epiblast layer. In the Sauropsida the tropho- 
blast is much less active than in Mammals, in consequence of the 
formation of an egg shell. The polyphyletic derivation of the three 
sub-classes of Mammals, which is a necessary supposition for the hypo- 
thesis here developed, is made probable by the palaeontological results 
of the last decade, indeed, may be said to be almost definitely established. 
* ‘ Phylogenesis of the Amnion/ Amsterdam, 1895, large 8vo. 66 pp. and 4 pis. 
