218 NIKOLAS K.LEINENBERG. 



sule, but then nearly always the remains of its companion 

 are found. 



This mode of reproduction appears to me worthy of some 

 remark, although it is not my intention to enter here into a 

 discussion of the known facts of development of other 

 animals which might be compared with it. Apparently in 

 our case there is not a succession of individuals, in which 

 only the first owes its existence to the co-operation of the 

 sexual elements, while the other takes its origin from it by 

 agamic generation ; from the egg of L. trapezoides two in- 

 dividuals arise directly and essentially independently of one 

 another. In the cases described last, in which a well-de- 

 veloped embryo produces the rudiment of the other, the 

 second should be considered to be a bud, but such a case is 

 abnormal ; regularly, the second embryo, although formed a 

 little later, and in connection Avith the other, does not de- 

 velop from the embryoplastic material employed in the 

 formation of the first, but from a portion of the blastomeres 

 derived directly from the segmentation which remains intact 

 until it becomes an independent formative centre. 



To interpret the division of the embryoplastic material as 

 the expression of a fission that happened at first in the adult 

 animal and then, in the course of generations, became put 

 back by the help of natural selection to the beginning of 

 development, would be to make a very arbitrary and little 

 satisfactory hypothesis, which also would be in antagonism 

 with the knowledge that we have of the fission and germina- 

 tion of the annelids. As far as we know, this process takes 

 place regularly in the posterior part of the body (not at the 

 head end), and this is not merely an empirical law, but is 

 explained by the fact that in many annelids the posterior 

 extremity retains during life distinctly embryonic characters. 

 Hence there is no more probable explanation of the double- 

 ness of the embryos than what can be found in the original 

 internal arrangement of the fecundated egg, a thing which 

 is not so strange, since the experiments of Haeckel on the 

 Siphonophoree ^ have shown the possibility of multiplying 

 the number of embryos by artificial division of the first mass 

 of blastomeres. Nevertheless, the case we have before us 

 appears to be without analogy in the development of other 

 animals. 



Todaro established, three years ago, that the individuals 

 of the compound stock of Salpa are to be considered, not as 

 children, but as younger brothers of the solitary stock ; how 



1 ' Zur Entwickelungsgeschichte der Siphonophoren.' Utrecht, 1869, 

 p. 73. 



