406 JOHN BEAKD, 



there is an abrupt passage ^) from the transitional stage, i. e. from the 

 larva, to the mature form. In others again there is a gradual sub- 

 stitution of a larval or asexual by an adult or sexual organism. 

 According to my interpretation of the facts, in the skate the change 

 would appear to be somewhat gradual, except at one point contem- 

 poraneous with the initiation of degeneration in the transient nervous 

 apparatus. 



Prior to that period the "embryo" had little likeness to a skate. 

 It bore more resemblance to the "embryo" of a shark or dog-fish 

 (ScyUium), and the latter is only similar to a dog-fish in being rounded 

 in form. To state the matter more plainly, the "embryo" is unlike 

 any existing adult organism , and the sum-total of its organisation 

 does not reflect the functional form of that of any existing organism, 

 some of its organs being as yet only in course of development, others, 

 like the sexual organs, only in an undififerentiated condition. Assuming 

 for the moment that the development were here an obvious antithetic 

 alternation of generations, where the sexual organism gradually re- 

 placed an asexual one, which, as in plants, was the seat of origin of 

 the former, what would conceivably be the course of events, when the 

 one organism gradually replaced the other? 



The two organisms would exist functioning together, until the one 

 outweighed, overcame, and then, finally, suppressed the other. What 

 Kleinenberg has demonstrated in cases of substitution of organs 

 would take place here. The one organism, the sexual form, originating 

 upon, not from, an asexual generation, would gradually encroach more 

 and more upon the latter, and, when it had attained a certain degree 

 of functional value, it (the sexual form) would compel the suppression 

 and extinction of the asexual generation. 



And this, according to my view, is precisely the natural inter- 

 pretation to be placed upon the facts in the instance under discussion. 



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

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

 The development only differs in essentials from that of plants ^) in 

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



1) Another striking instance is that of the "metamorphosis" of 

 Actinotrocha into Phoronis. 



2) This is, of course, if one includes the blastoderm. 



3) The conclusions here outlined were originally arrived at in- 

 dependently of any considerations as to the nature of plant-development. 

 The factors of animal-development alone led to a suspicion, and then 

 to a recognition, of an antithetic alternation. 



