94 Studies in the Theory of Descent. 



proved some years ago by P. E. M tiller, 9 who 

 studied the ontogeny, that this last was direct, since 

 the embryo, before leaving the egg, already pos- 

 sesses the form, members, and internal organs of 

 the adult. This was, at least, the case with the 

 summer eggs. It was subsequently shown by 

 Sars 10 that this mode of development only holds 

 good for the summer brood, the winter eggs pro- 

 ducing an embryo in the spring which possesses 

 only the three first pairs of limbs, and, instead of 

 compound eyes, only a single frontal eye, thus 

 exhibiting briefly, at first, the structure of a Nau- 

 plius, and gradually acquiring that of Leptodora. 

 The mature form derived from the winter eggs is 

 not distinguishable from the later generations, ex- 

 cept by the presence of the simple larval eye, which 

 appears as a small black spot. The generations 

 when fully developed are thus distinguished only 

 by this minute marking, but the summer generation 

 undergoes direct development, whilst the winter 

 generation, on the contrary, is only developed by 

 metamorphosis, beginning with the simplest Crus- 

 tacean type, and thus fairly representing the phy- 

 letic development of the species. We therefore 

 see, in this case, the combination of a metamorphic 

 and a direct development taking place to a certain 



8 P. E. Muller, " Bidrag til Cladocerners Fortplantingshis- 

 torie," 1868. 



10 Sars, in " Forhandlinger i Videnskabs Selskabet i Christia- 

 nia," 1873, part i. 



