506 Experimental Studies on tlie Origin of the Lens in Ampliibia 



ment of tlie optic vesicle by a hot needle or an electric cauterizing needle. 

 This was done on the early stages of triton embryos before the closure 

 of the neural canal. At this time the beginnings of the optic vesicles 

 can be seen at the anterior end of the medullary plate. Their destruc- 

 tion usually involves more or less injury to the neighboring parts, espe- 

 cially the brain, but need not interfere with that portion of the epithe- 

 lium which normally gives rise to the lens. Spemann's experiments 

 show that in some instances the rudiment of the optic vesicle was so far 

 destroyed that its subsequent regeneration and growth were insufficient 

 to bring it into contact with the skin epithelium. In all such cases 

 where the optic vesicle remained deep, there was absence of lens forma- 

 tion on that side. If, however, the injury to the optic vesicle rudiment 

 was not sufficient to prevent the more or less regenerated optic vesicle 

 from touching the skin epithelium, a lens was formed at the point of con- 

 tact between the optic vesicle and ectoderm. 



Herbst ' likewise concludes from a study of normal and abnormal 

 specimens that the contact of the optic vesicle on the ectoderm is neces- 

 sary for lens formation. 



METHODS AND MATERIAL. 



In order to test these and other questions, I employed during the 

 spring of 1903 a quite different method from Spemann's, one which I 

 believe is capable of extension to many other organs and tissues. One 

 can, with the aid of a binocular microscope and very delicate instru- 

 ments, make exceedingly minute dissections of the living amphibian 

 embryo and can remove various organs, transplant them, or alter the 

 normal relations and so alter the influences they exert on each other. We 

 may thus determine certain correlations necessary to normal develop- 

 ment. Besides this modification of Born's ^ method I have found Har- 

 rison's ' method of grafting different-colored species of frog embryos 

 together (heteroplastic grafting) very useful. 



The embryos of Eana palustris and E. sylvatica were used for the ex- 

 periments. Serial sections 10 micro mm. in thickness were made of 

 the embryo in each experiment. They were stained in hgematoxylin and 

 Congo red. 



^ Formative Reize in der Tierischen Ontogonese, 1901, p. 60. 



^ Born, G. Ueber Verwachsungsversuche mit Amphibienlarven. Archiv 

 f. Entwickelungsmechanik, Bd. IV, 1896-97. 



* Harrison, R. G. The growth and regeneration of the tail of the frog 

 larva. Archiv f. Entwickelungsmechanik, Bd. VII. — Experimentelle Unter- 

 suchungen ueber die Entwickelung der Sinnesorgane der Seitenlinie bei den 

 Amphibien. Archiv f. mik. Anat. u. Entwickelung., Bd. 63, 1903. 



