THE PRINCIPLES OF EMBRYOLOGY 459 



ingrowths, that the heart arises as two longitudinal veins, jnst 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, Palaeontology, 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 



