MEMOIRS. 
On the Structure and Homologies of the Germinal 
Layers of the Embryo.^ By F. M. Balfour, M.A., 
F.R.S., Fellow of Triiiity College, Cambridge. 
The discovery by Pander and Von Baer that the young 
embryos of vertebrated animals were formed of distinct 
layers of cells known as the germinal layers, and that there 
were special and constant relations between these layers and 
the adult organs throughout the group, remained for a long 
time an isolated fact. In 1859 Huxley made an important 
step towards the explanation of the nature of these layers 
by comparing them with the ectoderm and endoderm of the 
liydrozoa. 
In spite of Huxley’s comparison it was for a long time 
the generally accepted view that germinal layers similar 
to those in the Vertebrata were not found amongst inverte- 
brated animals. The brilliant researches of Kowalewsky on 
the development of a great variety of invertebrate forms 
lii-st proved that the accepted view was erroneous, and they 
led Lankester (No. 16) and Haeckel (No. 9) to publish 
certain speculations which have had an incalculable 
influence in stimulating and directing modern embryological 
research.^ 
At the time when the essays containing these speculations 
were published there appeared to be every probability of 
a definite answer being given to the questions raised in 
them. The results of extended investigations during the 
last few years have, however, shown that these expectations 
were premature; and there are very few embryologists who 
* Tliis paper, with some modifications, forms part of a chapter iu tlie 
fortlicomiii{^ volume of my treatise on ‘ Comparative Embryology.’ 
^ If I do not refer to the above authors to any very great extent in the 
sequel, it is not because 1 undervalue the importance of their work, but 
because their view's have been so much before the public that it is quite 
unnecessary to discuss them in detail. 
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