NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 231 



parallel to the neural tube, toward van AVijhe's third somite. In later 

 stages the nerve divides into two branches, one passing along the inner 

 side of the somite, and the other along the outer side. At first the nerve 

 root, which appears as a plasma outflow from the neural tube, is of the 

 thickness of one, or at most two, medullary cells. Later the roots 

 increase in thickness, apparently by the continued outgrowth of plasma 

 from the neural tube, as well as by the migration of cells from the 

 ventral horn of the medulla. The larger size and different staining 

 qualities of the medullary cells enabled him to distinguish them from 

 the mesodermal cells in their vicinity. Such (medullary) cells are often 

 found with a part of the nucleus within and a part without the medul- 

 lary wall. This outflow (migration) of meduUaiy cells takes place also 

 in later stages after the white substance has become quite thick on the 

 side wall of the neural tube.-^ 



My observations upon the development of the abducens diff"er from 

 those of Dohru, as in the case of the oculomotorius, inasmuch as I find 

 the nerve to arise from axis cylinder processes of neuroblasts in the 

 ventral horn of the medulla, and therefore to resemble in its mode of 

 development that of a ventral spinal nerve, as stated by His ('89). At 

 the earliest stage which I have been able to detect the abducens, it 

 possesses but a single root, formed by the processes of several neuro- 

 blasts, as is represented in Figure J, The union of these takes place 

 just outside the medullary wall, yet peripherally the nerve appears 

 as a single process with deeply staining axis and a more lightly stained 

 sheath. I find neither at this stage nor in later stages any convincing 

 evidence of a migration of the neuroblast cells from the wall of the 

 neural tube. In later stages of development sections show that the 

 nuclei seen along the course of the nerve are distinctly peripheral in re- 

 lation to its fibres. Even the phenomena presented in sections of 

 embryos fixed with corrosive-sublimate acetic, such as are represented in 

 Figures 62-65 (Plate 9), warrant in my judgment only the inference 

 that the nuclei of the nerve are peripheral, as held by Miss Piatt ('91). 

 The darker appearance of the nuclei lying upon the nerve results more 

 from the opaqueness of the nerve than from any peculiar staining proper- 

 ties of the nuclei. During development the number of roots in the 

 nerve increases from one to three or four, the number being variable 

 even upon the two sides of the same embryo. The method of develop- 



1 Since Marshall ('81), van Wijhe ('82), and Miss Piatt ('91) never saw the early 

 stages of development of the abducens, it is unnecessary to restate their results in 

 this connection. 



