Ii8 Incubation of the Top- Minnow (Gambusia). [February, 



val. Otherwise we are at a loss to explain the enormous fertility 

 of many marine forms ; the astounding fertility of the oyster and 

 clam are other instances illustrating this principle, where ova are 

 matured by the tens of millions, but where barely one out of a 

 million survives so as to attain adult age. 



Certain adaptations of structure are also plainly noticeable on 

 a comparative study of fish ova. Thus the egg membrane of 

 floating eggs is extremely thin, thinner than that of heavy or 

 adhesive eggs, while the thickest membranes are those provided 

 with external filamentous appendages. The most thinly clad 

 hatch out soonest. May it not be that the thinness of the en- 

 velope of the egg has some relation to the rapidity with which 

 the oxygenation of the egg is effected, and consequently with the 

 rapidity of tissue and embryonic changes ? And, finally, who 

 would undertake to say that all of these modifications of the em- 

 bryonic envelope are not such as could be developed by natural 

 selection so as to favor the survival of the greatest number of 

 embryos ? 



Many other general views of a similar character might be 

 drawn from the material in my possession, but I fear that there 

 has been already too much detail entered into for this note to be 

 of interest to the general reader. 



Before closing I wish to state that it is the oviduct of the female 

 in some cyprinodonts that is prolonged into a tube at the anterior 

 edge of the anal fin. This difference, as compared with Gambusia, 

 would be useful as a generic character, as suggested by Colonel 

 Marshall McDonald, to whose unselfish, helpful interest I am 

 deeply indebted for assistance in manifold ways, while the in- 

 vestigation of the material was in progress, upon which the fore- 

 going account is based. 1 



a cyprinodont is that by M. Duvernoy, Sur le develof.pement de la Paecilia Surinam- 



alcoholic material, but shows the remarkable acceleration of development of the 

 embryos the same as in Gambusia. The number of embryos, their arrangement in 

 the ovary, and the position of the ovary itself appear also to be similar. 



