132 CHIM^EROID FISHES AND THEIR DEVELOPMENT. 



embryologically at least, indicate a primitive segmented condition (Redecke), and 

 I am led to suggest that the "segmentation" of the kidney of the adult arose 

 secondarily. The absence of the Geschlechtsniere in Chimseroids, moreover, I also 

 interpret as a secondary reduction, an accompaniment of the enormous develop- 

 ment of kidney in a short body cavity, a process which caused an enlargement of 

 functional nephric tubules, an obliteration of rudimentary ones, and more direct 

 and special means of carrying out gonadial products. In favor of the last inter- 

 pretation are the great size and elaborate regional differentiation of Wolffian and 

 Miillerian ducts. 



NERVOUS SYSTEM. 



The following features in the development of the system may be mentioned as 

 indicating that the Chimaeroids have been subject to wider changes than kindred 

 sharks. 



Rcdiiction of Cord. The cord in the region of tail and hinder trunk, repre- 

 senting about 60 per cent of its entire length, is greatly modified. Contrast in this 

 region the diameter of the cord, its histological differentiation, the size, number, 

 and character of the roots of the spinal nerves. The flattening of the cord in the 

 hinder trunk and tail region is, therefore, hardly to be compared to the condition 

 in Cyclostomes. 



Flexure of Brain. In Callorhynchus (cf. Schauinsland's Taf. xxn) the brain 

 shows extraordinary flexures; in the region of the midbrain its axis changes direc- 

 tion by almost 180. 



Size of Infundibrihim. In early stages the infundibulum attains great size; 

 and concomitantly the dorsal wall of the diencephalon is compressed between the 

 forebrain and the optic lobes. 



Separation of Hemispheres. In this regard the early condition is more marked 

 than in any other fish-like vertebrate. Observe also the separation of the entire 

 forebrain from the midbrain. This in Chimaera begins in early stages and in the 

 adult attains remarkable proportions. 



