Morphological Studies. 759 



Nervenwurzeln. Der Riechnerv entsteht also erst nach dem Acranier- 

 stadium, und in Uebereinstimmung damit ist seine Abwesenheit beim 

 Amphioxus.'''' 



This passage, along with the true statement of the origin of the 

 olfactory nerve as an epiblastic differentiation, contains a number of 

 theoretical conclusions which I do not support; all we have to do here 

 is to accept the one simple statement of the actual origin of the 

 nerve (better neural ganglion of the olfactory) and the denial that it 

 arises, as the neural ridge was supposed to do, as an outgrowth of 

 the central nervous system. 



Before making renewed investigations on the development of the 

 olfactory nerve and organ, in order to test van Wijhe's observations, 

 I came to the conclusion that there was a good deal of truth in what 

 he stated. For the results of a very extended and complete series of 

 observations on the early beginnings of the cranial and spinal ganglia, 

 part of which has already appeared (37), led to the recognition that 

 these ganglia are formed as epiblastic diflferentiations , and not, as 

 usually supposed by nearly all observers, as outgrowths of the central 

 nervous system. But this was not sufficient, for it became necessary 

 to examine once again whether, like other cranial segmental nerves, 

 there is also a double origin of ganglionic elements in the case of the 

 olfactory metamere. The result is given in the following lines. 



Figs. 22 and 24 (pi. XXXVIII) are from a section passing through 

 the anterior neuropore of a very young Elasmobranch embryo, the portion 

 of the lip of the „neuropore" marked by the closed figure in fig. 22 

 is drawn under high power in fig. 24. 



The epiblast of the reentering angle between epiblast and fore- 

 brain is seen to be very thick, and we shall see that it is this thicken- 

 ed portion of the epiblast which gives rise to a portion of the ol- 

 factory ganglion. I may here mention that later on this portion 

 becomes converted into the olfactory nerve. Most nerves in the Ver- 

 tebrata are transformations of ganglion cells, just as muscles are 

 transformations of muscle-cells. 



In fig. 17 (pi. XXXVIII) we have the appearances presented by a 

 transverse section through a young Torpedo embryo. The plane of section 

 passes through both fore brain {fb) and hind brain (hh). In the latter 

 region is seen a portion of the „Anlage" or foundation of the trige- 

 minal ganglion (F). 



The upper portion of the section shows the fore brain and the 

 optic vesicles, and the point where the fore brain reaches the epiblast 



