xxxiv Journal of Comparative Neurology. 



are completely closed and separated from the nervous elements. In 

 the ganglion layer karyokinesis is in progress and toward the periph- 

 ery the cells show branched processes. "While this differentiation is 

 continued caudad, the cephalic part of this proton, which has been 

 already formed, begins to atrophy after having previously separated 

 into the protons of the ganglia of the first cranial nerves. The whole 

 proton is now plainly differentiated into three parts ; the first is the 

 proton of the cranial nerves in front of the trigeminus, the second of 

 the trigeminus, the third subsequently separates into the acustico-fa- 

 cialis and the glossopharyngeo-vagus systems. The successive posi- 

 tions of these parts, their union with the nerve-tube not only dorsally 

 but laterally and the evidence of active karyokinesis in the latter, all 

 go to show that the ganglia are developed from the nerve-tube. The 

 existence of mitotic figures in the peripheral part of the proton shows 

 in turn that these elements are now morphologically separate and that 

 they continue to multiply independently. 



3. The formation of the embryonic nervous groups and their 

 ganglia. Five such groups are distinguished. (1). Those cephalad 

 of the trigeminus. This group is composed a small number of ele- 

 ments which are hard to separate from the next group. At its max- 

 imum development it consists of a loose group of cells above the optic 

 vesicles. The principal outgrowth of this proton passes behind the 

 eye, and there forms a large oval mass which then degenerates, after 

 having entered into relations with that part of the trigeminus proton 

 from which originates the ciliary ganglion. This connection, however, 

 is considered secondary and unimportant,. The whole proton of this 

 first group subsequently disappears and the author considers that it 

 has only a transitory significance. While the observations here ex- 

 pressed differ in several more or less essential points from those re- 

 cently published by Dohrn, Baird, Kupffer and others, yet there are 

 certain broad lines of agreement among them which are very gratify- 

 ing. Kupffer's recent papers, especially, should be read in this con- 

 nection. 



(2). The Trigeminus group. It is in this locus that the peripheral 

 system first appears. In Raja clavata of 12 days this proton appears 

 as a large plate symmetrically covering the mid-region of the brain. 

 As development proceeds it becomes more sharply separated from the 

 neighboring groups. At 20 days the mandibular and ophthalmic rami 

 are indicated. As the fourth ventricle expands and the medulla be- 

 comes differentiated from the mid-brain, the trigeminus proton is car- 

 ried laterad and caudad, still retaining, however, its primitive connec- 



