146 ARTHUR DENDT. 



parts is very obvious in the case of an advanced embryo of 

 Hinulia, as seen in sagittal section and represented in my 

 own fig. 28. It is also made very clear by Burckhardt's 

 admirable figure of the brain of the embryo of Lacerta (9), 

 in which, however, he does not represent any commissura 

 fornicis. 



(h) The Accessory Vesicle. 



The accessory vesicle (fig. 14, Ac. V.), which appears between 

 the tubules of the paraphysis and the end of the parietal stalk 

 in the early part of Stage K of Sphenodonj is extremely 

 difficult to account for. The histological character of its walls 

 and the absence of any communication of its cavity with those 

 of the surrounding organs seem to indicate an independent 

 origin. It is conceivable that it may be formed from the out- 

 growth which I have identified as the commencing paraphysis 

 at Stages N and O (figs. 6, 9, 11, Par.), and that the paraphysis 

 itself originates later, between Stages O and R ; but this view 

 appears to me to be highly improbable. 



The absence of blood-corpuscles, the large size of the cavity, 

 and, to a less extent, the nature of the lining epithelium, 

 prevent us from regarding it as a blood-vessel. It may 

 perhaps be a large lymphatic sac, but this again does not 

 seem very probable. It may be homologous with the accessory 

 vesicle or " parapineal organ '^ described by Ritter (30) in 

 Phrynosoma, but it is very doubtful if it is homologous 

 with the ^'parapineal organ ^' of Studnicka in the lamprey. 



Burckhardt (9) figures a small vesicle in a similar position 

 in Lacerta vivipara, and, following Leydig, terms it the 

 " Nebenscheitel-Organ." Unfortunately I have not seen 

 Leydig's work, and therefore am unable directly to compare 

 the organ as described by him with the transitory accessory 

 vesicle of Sphenodon. Judging from the observations of 

 Prenant (26), however, it seems to me hardly likely that the 

 accessory vesicle of Sphenodon is homologous with the 

 accessory parietal organs of Anguis fragilis, which are 

 supposed to be budded off from the '' epiphysis " (parietal 

 stalk), and, like the latter, have thick walls. 



