14 PUOCEEDINGS OF THE 



variety of cartilage represents the embryonic form. Word for 

 word, the early embryonic stage of the Vertebrate skeleton closely 

 resembles the stage reached in the Arthropod, as shown by 

 Limuhts, and again records unmistakably the past history of the 

 Vertebrate. 



80, too, with the whole of the prosomatic region ; the situation 

 of the old mouth, the manner in which the nose of the 

 Ceplialaspidian fishes arose from the Pala30stracan, are all shown 

 with vivid clearness by KiiplTer's investigations of the early stage 

 of Ammocates, while at the same time the closure of the oral 

 cavity by the septum shows how the oral chamber was originally 

 bounded by the operculum. Nay, further, the very formation of 

 this chamber embryologically was brought about by the forward 

 growth of the lower lip, just as it must have been if the chihiria 

 grew forward to form the metastoma. So, too, the study of the 

 embryo teaches that the branchia) arise as ingrowths, that the 

 heart arises as two longitudinal veins, just as the theory supposes 

 from the facts provided by Lhmdus and the Scorpions. 



No indication of the origin of the thyroid gland is given by the 

 studv of its structure in anj 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 comparison 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 con- 

 firmed by, the study of the cojloinic cavities and tlie 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 tlie excretory organs : it is not 

 the kidney of the adult animal which leads direct to the excretorj'- 

 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 Kecai)itulation which is the foundation stone of embryo- 

 logical 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, 

 PaliEontology, Anatomy, and Physiology. 



It cannot then be said that my theory contravenes this great 

 law of development, the Law of Ilecapitulation. "What, then, is 

 the objection ? It is that it disregards the germ-layer theory, a 

 theory which assumes that the origin of the Metazoa from the 

 Protozoa took place by the formation of a gastrula-form — Haeckel's 

 hypothetical Gastrsea — which gave a fixed and definite morpho- 

 logical origin to hypoblast, and that from that time up to the 

 latest animal development that hypoblastic layer has always 



