230 E. I. WERBER. 



cases one is uncertain, for here a structure was observed which 

 might perhaps be a deformed lens-bud, in twenty cases neither 

 an optic cup nor a lens can be observed, while in five cases the 

 optic cup is so small that there was no contact with the epidermis. 

 According to Spemann, however, in two of the latter cases the 

 condition is not clear, one of the embryos possessing (on the side 

 of excision) near the brain "ein dickwandiges Blaschen mit 

 kleinem Lumen," which Spemann is unable to identify, while in 

 the other case the region of epidermis which should have fur- 

 nished the lens, is "kaum merklich verdickt." 



The only warranted conclusion from these results is: The contact 

 of at least a fragment of the optic anlage is necessary to induce the 

 formation of a lens from the overlying epidermis. It is unwarranted 

 however, to conclude, as does Spemann (12, p. 38), "... dass 

 Bombinator pachypus zwar auch primare Linsenbildungszellen 

 besitzt, welche von den Epidermiszellen der Umgebung ver- 

 schieden und zur Umbildung in die Linse vorbereitet sind, dass 

 diese Zellen aber der Mitwirkung des Augenbechers bediirfen, 

 um in Aktion zu treten, zum mindesten in viel hoherem Masse 

 als die Linsenbildungszellen von R. esculenta." 



The results of other experiments in this species also point 

 decidedly to the incorrectness of Spemann's conclusions. Thus 

 the removal of the optic vesicle during or immediately after the 

 closure of the medullary folds resulted only in a thickening 

 ("Wucherung") of the epithelium in the region where normally 

 the lens should have arisen. In two cases, however, small rem- 

 nants of the optic vesicle developed into diminutive optic cups 

 lacking a lens, because mesenchyme had grown in between them 

 and the epidermis. Lentoid structures (two) were observed only 

 in one case (Figs. 410 and b, L'), which are probably due to a 

 stimulus from very minute remnants of the optic vesicle. 



These experiments would again seem to prove only that, as 

 Lewis ('04, 'oja and b) and Le Cron ('07) have found, a stimulus 

 from the optic vesicle on the epidermis of sufficient duration is neces- 

 sary for the latter to give rise to a lens. They do not prove, however, 

 the presence in this species of early predetermined lens-forming 

 cells requiring the stimulus from an optic vesicle only as a supple- 

 ment for full differentiation into a lens. 



