SECT. XI DEVELOPMENT 169 



based. All that our argument requires is, that, while 

 the positive evidence is strong, the difficulties should 

 not be insuperable. 



We have had to limit our remarks to the larval 

 history of Apus, as observations on its embryological 

 development are unfortunately wanting. We may, 

 however, here mention one or two facts in the embry- 

 ology of the Crustacea which bear upon our theory. It 

 is stated,^ for instance, that the median eye develops 

 from paired rudiments — an observation which lends 

 some support to our account of the origin of this 

 organ out of an anterior pair of eyes. Still more 

 important for our theory is the fact that, while in the 

 development of a few Crustacea there is a tendency 

 in the mesoblast to form paired, segmented mesoderm- 

 streaks, in the majority of cases the mesoderm cells 

 form irregular lacunar spaces.^ The significance of 

 these two facts taken together cannot be over-estimated, 

 that is, if we are right in assuming that the latter 

 method of development of the mesoblast is gradually 

 displacing the former, and is therefore coenogenetic. It 

 will be remembered how, from purely morphological 

 reasoning, we came to the conclusion that the greater 

 part of the coelom epithelium (the parietal layer) of the 

 original Crustacean-Annelid went to form the mem- 

 brane of the lacunar blood system of the Crustacea. 

 We find then this acquired rearrangement of the meso- 

 derm shifted back to the earliest embryonic stages. 



1 Grobben, " Die Entwickelungsgeschichte der Moina rectirostris."' 



2 Korschelt unci Ileider, " Lehrbuch der vergleichenden Entwicke- 

 lungsgeschichte der Wirbelloseu Thiere." 



