No. 2.] EMBRYOLOGY OF THE LIZARD. -355 



sich also als zu der gesammten Anschwellung der Grundschicht 

 gehorig (Taf. III., Fig. 62)." P. 157. 



Even in the rather compressed parts of the amphibian q.^% 

 this pressure of the notochord is hypothetical. The existence 

 of this originally bilateral arrangement of the nervous thicken- 

 ings, in so many different forms, under different embryonic 

 conditions, casts strong doubt on the explanation by pressure. 

 It is also a questionable method of morphological reasoning, by 

 which the original significance of a part is first explained by 

 deductions from its ultimate condition. Goctte, in his description 

 of a later stage, refers to the subject again as follows : — 



" Im Rumpftheile sind die seitlichen Anschwellungen soweit 

 zusammengeriickt, das sie als zwei mit ihren Randern unmittel- 

 bar zusammenhangende Bauche (Medullarplatten) erscheinen. 

 Im Kopftheile, welcher sich viel langsamer und in geringerem 

 Masse zusammcnzieht, bleiben die seitlichen Anschwellungen 

 mehr auf den Rand der Axenplatte beschrankt, wahrend ein 

 nach Breite und Dicke ansehnliches, nach unten konkav gebo- 

 genes MittelstUck die urspriingliche Einheit der ganzen Platte 

 gegeniiber ihrer Entvvickelung aus scheinbar getrennten Seiten- 

 halften im Rumpfthei.le hervorhebt." P. 165. 



This " MittelstUck" is the same part that I have described in 

 the lizard as the anterior medullary fold. So far from disprov- 

 ing the bilateral separateness of the nerve-thickenings, it seems 

 to me to strengthen that view. If there were no anterior union 

 of the lateral nerve-thickenings, we should have in this case a 

 nervous system without a homologue ; but an anterior commis- 

 sure is characteristic of the Platyhelminthes, and this affords 

 ground for a comparison. I have shown in the lizard that the 

 first nerve-fibres appear to develop in longitudinal bands along 

 the internal surface of the lateral nerve-thickenings, and are 

 likewise continuous with each other anteriorly. If we suppose a 

 change in the chronological order of development, whereby 

 the nerve-fibres should appear during the earliest growth of the 

 lateral and anterior thickenings and before these latter had 

 formed a tube, then we should have an appearance very similar 

 to that described in Carinella, except that Carinella has the 

 nerve-trunks lateral, while in the vertebrate embryo they are sub- 

 dorsal or dorso-lateral. In the head, so great a similarity of the 

 nervous systems of these two forms disappears, but the main 



