History of a transient nervous apparatus in certain Ichthyopsida. 413 



transient apparatus exhibits signs of degeneration, i. e. of being in a 

 somewhat vestigeal form. And in another section it is briefly shown, 

 that there exists no morphological resemblance whatever between the 

 transient and permanent nervous systems, and that the one cannot be 

 regarded as an earlier developed portion of the apparatus, which 

 succeeds it. 



The concluding section of the memoir treats, in a preliminary 

 fashion, of development by substitution of organisms, or antithetic 

 alternation of generations. 



Among other things an attempt is made to show how in the 

 development of the skate a sexual form (the skate embryo) arises by 

 substitution on an asexual foundation or larva, the former gradually 

 replacing the latter. 



There would appear to be, in disguised form, two generations 

 contained in what is usually designated "a young skate embryo". The 

 one generation (here the sexual one) begins to arise on the other 

 (the asexual one) at a very early period after the fertilisation of the 

 egg. At first the asexual form, or reduced larva, obtains the start, 

 and gets into a position to develop its nervous system. Meanwhile 

 the future sexual form has been making slow but sure progress in 

 its development, and, anon, a period arrives when the two cannot 

 co-exist as of equal functional value. As shown by commencing de- 

 generation of its nervous system, the asexual form, or larva, begins 

 to get suppressed, and in the skate-development this appears to be 

 brought to pass by a sudden spurt on the part of the sexual generation. 



Previously the organism apparent to our vision lacked many of 

 the most essential characters of the skate, and its sex could not be 

 determined. Now, and now for the first time, it becomes markedly 

 flattened, its pectoral fins commence their forward growth, its organs 

 are getting into something like the adult form and histological con- 

 dition, the cells of the cord and of the spinal ganglia are becoming 

 distinctly ganglionic, the permanent canal of the central nervous 

 system is in course of formation, and, of some import, the sex of the 

 organism becomes obvious even from the external characters. 



These points indicate the great changes in operation, changes all 

 tending towards the attainment of the adult form. The thesis is 

 stated that all Metazoan development from the egg appears to be 

 fundamentally an antithetic alternation of generations, and that directly 

 or indirectly there is invariably a substitution of organisms. 



