HISTOGENESIS OF THE NERVOUS SYSTEM 



39 



vious relation with the other sensory nerves. They assume that the part of the neural crest, 

 which lies rostral to the anlage of the semilunar ganglion, fails to separate from the neural 

 tube. From this part of the neural crest, retained within the brain, they would derive the 

 mesencephalic nucleus of the trigeminal nerve and the optic vesicles. 



On the other hand, there are observations which tend to show that some of the cranial 

 sensory ganglia are derived at least in part from other sources than the neural crest. This 

 is especially true of the acoustic ganglion (Streeter, 1912). According to Landacre (1910) 

 many of the sensory ganglion cells of the seventh, ninth, and tenth nerves are derived from 



Fig. 20. A, Transverse section through the spinal cord of a chick embryo of the third day 

 showing neuraxons (F) developing from neuroblasts of the neural tube and from the bipolar 

 ganglion cells, d. B, Neuroblasts from the spinal cord of a seventy-two-hour chick. The three to 

 the right show neurofibrils; C, incremental cone. (Cajal, Prentiss-Arey.) 



thickened patches of the superficial ectoderm, known as placodes, with which the ganglia of 

 these nerves come in contact at an early stage in their embryonic development. The 

 acoustic ganglion of the eighth nerve seems also to have a similar origin, i. e., from the cells 

 of the otic vesicle which is formed by a process of invagination from the superficial ectoderm. 



The neuroblasts of these ganglia become bipolar through the development 

 of a primary process at either end (Fig. 21). Originally bipolar, a majority of 

 these sensory neurons in the mammal become unipolar through the fusion of 

 the two primary processes for some distance into a single main stem. Beyond 

 the point of fusion this divides like a T into two primary branches, one of which 



