THE PRINCIPLES OF EMBRYOLOGY 459 
ingrowths, that the heart arises as two longitudinal veins, just as the 
theory supposes from the facts provided by Limulus and the scorpions. 
No indication of the origin of the thyroid gland is given by the study 
of its structure in any adult vertebrate, but in the larval form of the 
lamprey there is still preserved for us a most graphic record of its 
past history. 
The close comparisons which it is possible to make between the 
eye-muscles of the vertebrate and the recti muscles of the scorpion 
group on the one hand, and between the pituitary and coxal glands on 
the other, are based upon, or at all events are strikingly confirmed by, 
the study of the ccelomic cavities and the origin of these muscles in 
the two groups. In fact the embryological evidence of the double 
segmentation in the head and the whole nature of the cranial 
segments is one of the main foundation-stones on which the whole of 
my theory rests. 
So it is throughout. Turn to the excretory organs—it is not the 
kidney of the adult animal which leads direct to the excretory organs 
of the primitive arthropod, but the early embryonic origin of that 
kidney. 
So far from having put forward a theory which runs counter to 
the principles of embryology, I claim to have vindicated the great 
Law of Recapitulation which is the foundation-stone of embryological 
principles. My theory is largely based upon embryological facts, and 
its strength consists in the manner in which it links together into 
one harmonious whole, the facts of Embryology, Paleeontology, Ana- 
tomy, and Physiology. Why, then, is it possible to assert that my 
theory disregards the principles of embryology, when, as we have 
seen, embryology is proclaiming as loudly as possible how the verte- 
brate arose? In my opinion, it is because the embryologists have 
to a large extent gone wrong in their fundamental principles, and 
have attached more weight to these faulty fundamental principles 
than to the obvious facts which, looked at thoughtfully, could not 
have failed to suggest a doubt as to the correctness of these 
‘ principles.’ 
The current laws of embryology upon which such weight is laid 
are based on the homology of the germinal layers in all Metazoa, and 
state that in all cases after segmentation is finished a blastula is 
formed, from which there arises a gastrula, formed of an internal 
layer, the hypoblast, and an external layer, the epiblast ; subsequently 
