History of a transient nervous apparatus in certain Ichthyopsida. 371 



The degeneration of the transient nervous apparatus. 



This will be followed step by step in a series of twelve embryos 

 whose sizes range from 8,7 cm to upwards of 19 cm, the latter being 

 the length attained by three jB. batis embryos which were reared until 

 they hatched. The total period of the development to this stage 

 covered 17 — 18 months. In other words the fresh eggs were obtained 

 from the female skate in March and April 1890, and the young ones 

 left the purse in the early days of September 1891. Newly laid skate- 

 eggs are only in the first stages of segmentation. 



9. Embryo of 8,7 cm »)• 



The embryo (No. 261) is six months old. Under the conditions 

 in which it was being kept it was nearly a year distant from the 

 hatching period. The length of the body from the rostrum to the 

 end of the pelvic fins is 23 mm, that of the tail 64 mm, and the 

 total 8,7 cm. The breadth across the widest part of the pectoral 

 fins is 17 mm from side to side. The specimen is a female. The 

 external gills are very long. 



There are three figures from this embryo on plate 28 (Figs. 106 a 

 and b and 107). 



As will be seen, in the two former the permanent central canal 

 is in course of formation, and the transient ganglion-cells (glee) are 

 being jammed into the posterior fissure. This will be more obvious, 

 and may then be commented upon, in somewhat later stages. The 

 processes of the quondam multipolar ganglion-cells have withered, and 

 the cells show a tendency to assume a rounded but irregular shape. 

 The cell- protoplasm is no longer so granular. But little change has 

 yet come about in the nucleus and nucleoli. From the same embryo 

 two degenerating ganglion-cells (w. gl c) in the mesoderm just outside 

 the cord are sketched in Fig. 107. Their condition resembles that 

 of their central fellows, and they are no longer connected with nerves, 

 or, in other words, they are ganglion-cells without nerve-processes. 



Another figure from embryo No. 261 is Fig. 115 of plate 29. 

 Four of the central cells as seen under high magnification are de- 

 picted. The short processes of the cells and the vacuoles {v) in the 

 lower one are the points specially to be noted. 



1) The size attained by an embryo H. hatis in a given time is, of 

 course, a function of the temperature of the sea-water. 



Zool. Jahrb. IX. Abth. f. Morph. 25 



