HISTORICAL SKETCH 5 



was doubtful of its nature and even contended that it was not a sense- 

 organ. Goette (1875), described the epiphysis in Bombinator and stated 

 that the site of its out-growth from the brain was identical with the position 

 of the anterior neuropore, or the point where the medullary folds finally 

 unite with each other in the formation of the neural tube. It was, how- 

 ever, shown by Mihalkovics that at the time of the first appearance of the 

 evagination in birds (chick) (Fig. 200, Chap. 21), and in mammals (rabbit) 

 (Fig. 208, Chap. 22), the medullary wall is separated by a considerable in- 

 terval from the cutaneous ectoderm, and that the intervening space is filled 

 in all amniote embryos by a layer of mesenchyme long before the pineal out- 

 growth takes place ; and it has also been shown by Kupffer in anamniota 

 that the pineal evagination in Acanthias embryos lies behind the point of 

 closure of the anterior neuropore and arises independently of it (Fig. 2). 



Fig. 3. — Free-swimming Larva of Phallusia or Ascidia Mammillata, showing 

 the Single Eye and Otocyst enclosed within the Cerebral Cavity 

 (ventricle). 



adh. pap. : adhesive papillae. med. : medullary tube. 



al. c. : alimentary canal. n. ch. : notochord. 



atr. : atrial opening. ot. cy. : statocyst. 



oil. gr. : ciliated groove or funnel. sens. ves. : sensory vesicle. 



end. : endostyle. stig. : earliest stigmata. 



eye : right eye. 



These specimens illustrate the production of a single eye or a single stato- 

 cyst by the suppression (complete or incomplete) of one member of a pair of 

 sensory organs. 



(From Korschelt and Heider, after Kowalewsky.) 



Gotte's opinion with regard to the formation of the pineal diverticulum 

 at the point where the roof of the brain remains latest attached to the 

 external skin was also criticized by F. M. Balfour in 1885, who stated that 

 he could find no indication in elasmobranchs of a process similar to that 

 which was described by Gotte, and that his observations had not been 

 confirmed for other vertebrates. Balfour also alludes to Gotte's com- 

 parison of the pineal gland or diverticulum to the " long-persisting pore 

 which leads into the cavity of the brain in Amphioxus," and he comments : 

 " We might also add that of the Ascidians " (Fig. 3). 



