MEMOIRS. 
A Comparison of the Eariy Stracss in the DEVELOPMENT 
of VerTeBRATES. By F. M. Batrour, B.A., Fellow of 
Trinity College, Cambridge. (With Plate X.) 
Ir the genealogical relationships of animals are to be mainly 
or largely determined on embryological evidence, it becomes 
a matter of great importance to know how far evidence of 
this kind is trustworthy. - 
The dependence to be placed on it has been generally 
assumed to be nearly complete. Yet there appears to be no 
& priori reason why natural selection should not act during 
the embryonic as well as the adult period of life ; and 
there is no question that during their embryonic existence 
animals are more susceptible to external forces than after 
they have become full grown: indeed, an immense mass 
of evidence could be brought to show that these forces 
do act upon embryos, and produce in them great alter- 
ations tending to obscure the genealogical inferences to 
be gathered from their developmental histories. Even the 
time-honoured layers form to this no exception. In Elas- 
mobranchs, for instance, we find the notochord derived from 
the hypoblast and the spinal ganglia derived from the invo- 
luted epiblast of the neural canal, whilst in the higher 
vertebrates both of these organs are formed in the mesoblast. 
Such instances are leading embryologists to recognize the 
fact that the so-called layers are not quite constant and must 
not be absolutely depended upon in the determination of 
homologies. But though it is necessary to recognise the fact 
that great changes do occur in animals during their embryonic 
life, it is not necessary to conclude that all embryological 
evidence is thereby vitiated ; but rather it becomes incum- 
bent on us to attempt to determine which embryological 
features are ancestral and which secondary. For this pur- 
pose it is requisite to ascertain what are the general 
characters of secondary features and how they are pro- 
VOL. XV.—NEW SER. P 
