244 



Dation brings about are far too numerous and too varied to admit 

 of treatment here. It would be necessary, even with the facts already 

 available, to write a treatise on animal development from this stand- 

 point, in order to display them. 



But it may be of interest to indicate one or two developmental 

 facts, which clearly have their natural interpretation in an aposporous 

 formation of the sexual generation in the Metazoa. 



The marine Annelida with an obvious larval development admit 

 readily of inclusion in such a scheme as that suggested. Kleinen- 

 BEE«^) in his brilliant Lopadorhynchus memoir has amply 

 demonstrated such an alternation as that here recognised , while just 

 failing to draw the manifest conclusion. By way of parenthesis it 

 may be remarked how marvellously close on a recognition of this 

 "law of development" Kleinenbeeg's meditations, along with those of 

 Johannes Müller and von Baer, really verge. 



A Chaetopod origin of the group of Hirudinea is commonly 

 admitted, and in them R. S. Bergh has demonstrated facts in his 

 memoirs furnishing as valuable confirmation of the views here advoca- 

 ted as could be wished. 



Passing next to the group of the Oligochaeta, we are appar- 

 ently brought to a standstill in our further search for confirmation. 

 But not really so. When we take up the researches of E. B. Wil- 

 son -), these at first sight seem hopelessly at variance with any such idea 

 as that of alternation of generations with apospory. It was longago foreseen 

 that here obstacles seemed to block the way. However, when looked 

 at in the light of spore-formation, Wilson's lines of cells, mesoblasts, 

 neuroblasts etc. readily admit of interpretation, not as due to an 

 actual spore-formation , but as an early modification of this , which 

 has already led some distance along the path of apospory. Really as 

 a step in advance from the former formation of the sexual generation, 

 or gametozooid, from a spore-mother-cell, in the direction of its origin 

 from a few cells. Carry the process still further, and we obtain the 

 counterpart of the primitive streak of the Vertebrata. It is very 

 interesting to note that, altogether apart from theoretical considera- 

 tions, AsSHETON points out how in the embryology of the frog and 

 rabbit the first attempts at development result in products formed in 

 a totally different direction from that subsequently adopted. 



1) Zeitsehr. f. wiss. Zoo)., Bd. 44, 1886. 



2) E. B. Wilson, The Germ-bands of Lumbricus. Journ. of Mor- 

 phology, Vol. I, 1887. 



