History of a transient nervous apparatus in certain Ichthyopsida. 407 



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

 of the egg. The two are never separate, but are so interblended that 

 it is a matter of great difficulty to separate analytically the one from 

 the other. 



At first the asexual form , in a reduced condition , obtains the 

 start, and gets into a position to develop its nervous system, — a 

 system , as we have seen , totally difierent in characters from the 

 nervous system of a Vertebrate animal. But, meanwhile, the future 

 sexual form has been making slow but sure progress in its develop- 

 ment, and, anon, a period arrives when the two cannot co-exist as of 

 equal functional value. As shown by commencing degeneration of its 

 nervous system, the asexual form begins to get suppressed, and in 

 the skate-development this appears to be brought about 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 very mar- 

 kedly flattened. Its pectoral fins commence their growth forward, and 

 its organs are getting into something like the adult form and histo- 

 logical condition. Thus, the thymus is now free from the gill-clefts, 

 the cells of the cord and of the spinal gangha are becoming distinctly 

 ganglionic, the permanent canal of the 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 alone suf- 

 ficiently emphasise the great changes in operation, changes all tending 

 towards the attainment of the adult forra^). 



1) It appears to be an universal rule in animal-development that 

 the alimentary canal of the larva should be carried over into the service 

 of the adult organism. A curious phenomenon in Vertebrate develop- 

 ment, and probably generally present, is the closure of the embryonic 

 oesophagus, and its persistence in this condition for a prolonged period 

 of the development. This remarkable fact has been commented upon 

 by Balfour and by Marshall and Bles , but no adequate attempt to 

 explain it has ever been made. According to my view the matter is 

 quite simple. The larval gut is not primarily adapted to serve as the 

 alimentary canal of a Vertebrate, and it closes either because it is no 

 longer functional in the larva, or because it requires to be, as it were, 

 rebuilt before it can serve as the gut of the Vertebrate. Both reasons 

 may possibly be at the bottom of the anomaly, de Meubon has offered 

 a solution which, if acceptable, would only apply to Mammals, for it 

 evades, though noting it, the circumstance that the phenomenon is of 

 general occurrence among Vertebrata. The reasons assigned by him to 



