Further Examination of the Cell-Theory SU^ 



examinations sufficiently searching. 



Tlie most extensive studies in this field have pertained to 

 groups of animals whose economic or ecological relations 

 to man are such as to render it important to recognize the 

 different stages of their lives. Thus in the interest of the 

 great marine fishing industries of northern Europe, elab- 

 orate investigations have been undertaken for identifying 

 the eggs and embryos of numerous species of fishes. Worthy 

 of special consideration is the partnership work by Fr. 

 Heincke and E. Ehrenbaum. 



After asserting the indispensability of exact specific de- 

 termination of floating eggs and young fishes as a basis for 

 any reliable deductions concerning the distribution of the 

 eggs, the authors say that the possibility of such deter- 

 mination can be affirmed only conditionally ; and their re- 

 search had for its object to show in general how far the 

 identifications can be made, and in particular to ascertain 

 the extent to which size is distinctive of the different species. 

 Agreeing with seemingly all zoologists who have attended to 

 the matter, they say that the oil drops and pigmentation 

 occurring in so many floating eggs furnish important dis- 

 tinguishing marks. The time of escape of the embryo from 

 the Qgg membrane seems also to be distinctive for many 

 species. Concerning pigment, they affirm that in the ad- 

 vanced embryonal stages, nearly all fish species can be rec- 

 ognized with great certainty. Included in the elaborate 

 study is a "table for determining the floating fish eggs in 

 the German North Sea." ^^ This is a "key" in the ordinary 

 sense of the taxonomist and deals with some thirty species 

 belonging to about twenty genera. The attributes used 

 chiefly in constructing this key are found in the oil drops, 

 pigment spots, and size of the eggs. 



Mosquitoes are another group of animals which have 

 drawn considerable attention to their early developmental 

 stages because of their importance to man; and here again 



