HISTOLOGY OF SENSORY GANGLIA OF BIRDS 253 



tional forms, from oppositi-polar to unipolar, was described and 

 figm'ed. The bipolar elements were shown to persist in large 

 numbers in the sensory ganglia of lower vertebrates and the uni- 

 polar derivatives to be characteristic of the ganglia of mature^ 

 higher vertebrates. 



His ('86) first made known the condition in the human embryo 

 by finding the cells of the spinal ganglia to be bipolar up to the 

 ninth week of development and, thereafter, gradually to change 

 into unipolar cells with a T-shaped process. 



Multipolar cells now began to be described in sensory ganglia 

 as: Disse ('93) frog; Cajal ('93) and von Lenhossek ('94) in spinal 

 ganglia of chick embryos; Spirlas ('96) goat embryos; Dogiel 

 ('96) adult mammals. Huber, also, in 1896, described a new kind 

 (spinal ganglia of frog, turtle, and embryo chick) with accessory 

 processes springing from the cell-body and terminating, frequently, 

 with bulb-like enlargements, within the capsule. He suggested 

 that multipolar cells in sensory ganglia are, probably, from the 

 sympathetic. 



Passing over much detail, the next general step was the 

 identification of numerous types of cells in the sensory ganglia 

 of mammals. Dogiel in 1896-97 described a new variety of 

 cell (type ii) with fine processes producing arborizations within 

 the ganglion, but without a distinct axon. Lugaro('94) indi- 

 cated five distinct types, and in 1908 Dogiel made an elaborate 

 classification embracing eleven positive types and intermediate 

 gradations. The eight morphological varieties of Cajal ('06) are 

 more easily recognizable than the types of Dogiel. 



Turning now from vertebrates in general to researches on the 

 sensory ganglia of birds we find eleven publications, of which only 

 the salient points will be indicated. 



Retzius ('80) found two kinds of unipolar cells in the spinal 

 ganglia of the chick; one with, and one without the medullated 

 processes. Both kinds were without glomeruli. The nerve ele- 

 ments of the Gasserian were found to be essentially similar to 

 those of the spinals. 



Rawitz ('82) in a comparative study of the ganglia of all classes 

 of vertebrates, observed the structure of the spinal and Gasserian 



