486 



a limitation in this respect that the organ becomes of service to the 

 organism only during early life. The higher we ascend in the Verte- 

 brate scale the more do its functions become restricted to the period 

 of youth of the organism, the more are they usurped by other lymph- 

 forming structures in maturity. As the guardian of the respiratory 

 organs the thymus is functionally relieved by the tonsils (palatine, 

 pharyngeal, etc.) in the Sauropsida and Mammalia, the change in the 

 organs of respiration. 



These views are in their very nature nothing more than pro- 

 babilities , but as Hatschek sen tenuously remarks , "the results of 

 Comparative Embryology have always only the value of probable con- 

 clusions, in exactly the same degree as those of Comparative Anatomy. 

 The relative certainty depends in both cases only on the number of 

 the premisses and on the exactness (Schärfe) of the conclusion" 1 ). 



Nachdruck verboten. 



The Mid-Brain and the Accessory Optic Vesicles. 



A Correction. 

 By William A. Locy. 



My preliminary Communication on the Derivation of the Pineal 

 Eye, printed in this Journal 2 ), contains an error that I desire to 

 correct. The error does not affect the main contention in that article, 

 viz. — that the pineal outgrowth is formed from patches of primitively 

 visual epithelium, or "accessory optic vesicles". It is merely a wrong 

 identification, in certain stages, of the first formed vesicle of the 

 mid-brain. 



In working out the details of the formation of the pineal outgrowth 

 in Squalus acanthias, I find that the surface contours of the head are 

 considerably altered, by the distribution of the mesoblastic cells, lying 

 between the external layer of epiblast and the brain walls. The meso- 

 blast forms a pad of varying thickness in close contact with the brain 

 walls; the depressions are filled, and in some places, thick patches of 



1) B. Hatschek, Lehrbuch der Zoologie, 1888, p. 26. 



2) Änat. Anz., Bd. IX, Nos. 5 u. 6, p. 169—180. 



