THE LENS-PROBLEM. 223 



in R. esculenta. He attributes this to a difference in the "Kon- 

 sistenz" between the embryos of the two species. "So ist wohl 

 das Resultat zu erklaren, dass der Defekt bei den allerdings 

 nicht sehr zahlreichen Versuchen dieser Art meist entweder zu 

 gross wurde, im ersteren Fall also vom rechten Auge ein Rest 

 erhalten blieb, im letzteren die Linsenanlage mit zerstort wurde." 



In my estimation the divergent results obtained both by prick- 

 ing with a heated needle and by excision in both species would 

 rather seem to point to differences in the location of the optic 

 pits in the neurulae of the two species at the same stage of de- 

 velopment. Thus, in Rana esculenta they are probably more 

 diffuse and extend more laterally than they are in R. fusca. 

 Accordingly, if at this stage the excision of the anterior half of 

 the brain primordium be attempted, it may happen that a small 

 fragment of the optic pit may remain, as in this species it may 

 extend even laterally from the medullary fold. Granting, how- 

 ever, that fragments of potential optic-cup substance do some- 

 times remain after the operation in this species and Spemann 

 admits that both in text and in the figures it is no longer difficult 

 to understand that in such embryos lentoid structures and even 

 well-differentiated lenses may be formed on the eyeless side owing 

 to the " lentogenic stimulus MI from the remnant of optic substance 

 on the epidermis. 



Spemann does not seem to have considered the results obtained 

 from these operations as very conclusive, if as much can remain 

 of the brain anlage after its attempted elimination as "ein 

 Fragment des Tapetum nigrum," "der vorderste Teil der Vor- 

 derhirnanlage, . . . ein dorsales Stiickchen Zwischenhirn mit 

 Epiphyse und Plexus chorioideus, jedenfalls entstanden aus 

 lateralen Partien der Medullarplatte, endlich ein Stiickchen der 

 linken Halite des Mittelhirns" ('12 p. 28). For, not content 

 with these results he sought confirmation in experiments in which 

 several other methods were employed. 



Thus, adopting Lewis's ('04) method, he removed (by excision) 

 in R. esculenta the optic vesicle after having previously raised up 

 and reflected the overlying epidermis. The latter was then 

 affixed in its place where it healed and in two embryos gave rise 



1 Cf. Werber ':6c. 



