136 SUMMARY OF CURRENT RESEARCHES RELATING TO 



make a common origin very doubtful. On the other hand, the author 

 believes that the appearances are readily explained on the hypothesis 

 that in Pfliiger's tubes in the embryo the primordial ova, instead of 

 being isolated with their follicular cells by the ingrowth of the con- 

 nective-tissue, were left in groups round which the sheath of connective- 

 tissue formed. 



Peripheral Nervous System of Salmo salar.* — Dr. E. G. Harrison 

 finds that in the development of the nerves of the salmon, the nerves 

 clearly originate from single cells, there is no indication of the occur- 

 rence of chains of cells. The spinal ganglia arise from wandering 

 cells which separate themselves from the medullary cord, and, migrating 

 ventrally, arrange themselves in little clusters which form the ganglia. 

 These are therefore not primitively metameric. For a long period the 

 cell-clusters remain undifferentiated, the cells then become bipolar, and 

 send one of their prolongations upward to the medullary cord. The 

 medullary cord consists chiefly of epithelial cells with a central zone 

 of "germ-cells" (His), and a peripheral zone of neuroblasts. The 

 medullary canal arises by the fusion of intracellular vacuoles which 

 arise within the central cells. Most of the neuroblasts become pear- 

 shaped, and send out a long process which forms a nerve-fibre. The 

 " posterior cells " (giant-cells of Bohon) arise in the dorsal part of the 

 cord, give rise to one or two prolongations, and migrate towards the 

 centre of the cord, the prolongations remaining in the original position. 

 Of these prolongations or fibrils certain become metameric peripheral 

 nerves, the majority are confined to the cord. The posterior cells de- 

 generate as the yolk-sac disappears, and are to be regarded as the 

 homologues of the spinal ganglion-cells, which in the later stages of 

 development take over their functions. They are to be compared with 

 the " transient ganglion-cells " of Beard in Rata. 



Development of Peripheral Nerves.t — Prof. F. Eaffaele has studied 

 the structure of the nerves in embryos of Lophius and in larval Am- 

 phibians. He finds that the evidence clearly points to their origin 

 from a chain of cells united by their extremities, or perhaps rather from 

 continuous protoplasm in which the cells are represented by their nuclei. 

 He figures the developing nerves showing the nuclei imbedded in the 

 protoplasm. Elongation of the nerves is produced by the mitotic divi- 

 sion of the nuclei, followed by an increase in length of the cells. A 

 striking figure shows the bifurcation of a developing nerve ; in it the 

 products of nuclear division lie side by side instead of end to end, and 

 the protoplasm has split into two threads. 



Development of Sympathetic System. J — Dr. C. K. Hoffmann has 

 investigated the development of this system in Acantliias vulgaris, as a 

 type of Selachians. Like Balfour and Van Wyhe, he finds that tbe 

 sympathetic ganglia originate as little swellings on the rami ventrales 

 of the spinal nerves. These swellings become differentiated into two 

 parts, of which the one consists of large nerve-cells and forms the sym- 

 pathetic ganglion proper ; while the other and larger part, which 



* Arch. Mikr. Anat., lvii. (1901) pp. 354-444 (3 pis. and 7 figs.). 



t Anat. Anzcig., xviii. (1900) pp. 337-44 (11 figs.). 



X Verh. K. Akad. Wet. Amsterdam, vii. (1900) pp. 1-80 (3 pis.). 



