164 KICHARD ASSHETON, M.A. 



des Augenblasenstieles ; eine secundare Losung der oberen Wand 

 vom Pigment-Blatt der Retina und eine Verschmelzung derselben mit 

 der eigentlichen Retina anlage findet nicht statt. Praparate von 

 Reptilienembryonen (Lacerta muralis und Tropidonotus natrix), welche 

 mit Boraxcarmin gefarbt und mit Picrinsaure nachbehandelt sind, zeigen 

 ferner, dass, bei diesen Thiere wenigstens, die ersten Sehnervenfasern 

 von der Peripherie centralwarts wachsen. Es ist demnach hochstens 

 wahrscheinlich, dass sie aus der Retina-anlage hervorwachsen, wenn 

 ihr Ursprung dort auch nicht direkt nachgewiesen werden konnte." 



The suggestion that the fibres of the optic nerve either develop 

 from cells within the retina and grow towards the brain, or that they 

 develop in the brain, and grow towards the retina, and are not 

 formed by the transformation of the cells of the optic stalk, was first 

 discussed as a matter of theory by His in the year 1868. 



His (10) at that time maintained, as he has so ably demonstrated in 

 a more recent work (11), that nerve-fibres are outgrowths from nerve 

 cells ; and holding that there are no nerve-cells along the optic stalk, 

 he suggested that the fibres of the optic nerve must grow from cells 

 probably in the brain along the optic stalk to the retina. 



With him in the main agreed W. Miiller (20), Mihalkovics, and 

 Kolliker, though they differed as to the direction of the growth of the 

 fibres. 



Certain other embryologists, however, were not convinced by what 

 was little more than suggestion ; among them was Balfour (1, p. 493), 

 who wrote, "There does not seem to me to be any ground for 

 doubting (as has been done by His and Kolliker) that the fibres of 

 the optic nerve are derived from a differentiation of the epithelial cells 

 of which the nerve is at first formed ;" and Balfour's opinion seems to 

 have been held until the present time by nearly every English writer 

 on the subject. 



There are apparently still those who hesitate to accept the more 

 recent views as to the origin of the optic nerve. As examples I will 

 quote passages from the last editions of two widely read educational 

 works, namely, 'A Text-book of Physiology,' by Foster, and 'The 

 Frog,' by Marshall. 



On p. 1141 of the former work Professor Foster writes: "The cup 

 becomes what we may speak of broadly as the retina, and we may 



