56 



THE MUSEUM. 



ther still in the use or abuse of food- 

 yolk in embryonical science. It has 

 even been made the basis of a phylo- 

 genetic tree of vertebrate ancestry. 

 What more could be asked of it than 

 this? The discoverer of this tree has 

 indeed disowned and rejected the fig- 

 ment of his own imagination; but, this 

 notwithstanding, from time to time it 

 tries to blossom forth anew. 



If it could be proved that, as we as- 

 cend the vertebrate scale, the food- 

 yolk either increased or decreased in 

 a regular and intelligible fashion, good 

 reason might underlie its use in phylo- 

 genetic speculation. But if we at- 

 tempt to evolve dog-fish and skate 

 from lampreys by increasing the 

 amount of food-yolk in the egg, gano- 

 ids from the former by again reducing 

 it, and amphibians from these by a 

 still further reduction, and by a new 

 and enormous increase reptiles from 

 amphibians, and so on to the end of 

 the mammalian chapter, we are not 

 really drawing upon an actual tangible, 

 but limited, supply of food-yolk, but 

 simply and solely on the intangible 

 and unlimited resources of the imagin- 

 ation. 



If it is suspected that the food-yolk 

 of an egg has either increased or de- 

 creased, some cause must have been 

 behind the change. It is only in fairly 

 tales, such as that of "Aladdin and 

 the Wonderful Lamp" that things 

 come into existence from nowhere and 

 out of nothing. As in physical science 

 so also in natural science every effect 

 has a cause and every effect is govern- 

 ed by a law or laws. 



If the food-yolk of an egg can be 

 shown to have increased in amount, 

 we must not forget that the egg has 

 had a history, in past times, and that 

 a plus added to it at or during some 

 epoch of time necessarily entails a 

 minus or abstraction from something 

 else at the same time. 



That this must be so may not be 

 obvious at the first glance. But it Is 

 all but certain that the cells in tiio 

 oviary which feed the ovarian egg (foll- 



icle cells) or in other cases the ovarian 

 cells consumed by the ovarian egg, are 

 themselves rudimentary or degenerate 

 or sterile eggs. And the amount of 

 nourishment they can furnish in any 

 given instance would seem to be defi- 

 nite and limited. If the egg of an an- 

 imal increase in size and amount of 

 food-yolk it may in all probability 

 rightly be concluded that fewer eggs 

 have been laid, and that in some form 

 or other more incipient eggs have been 

 used up in the process of forming the 

 functional one. 



This conclusion, or suspicion, brings 

 us natnrally to the main points of our 

 inquiry, i. e. to the modes in which 

 food-yolk has been obtained, and an 

 egg-capsule for the reception of a 

 single egg evolved. 



The simplest eggs are admittedly 

 those, like the eggs of many echinod- 

 erms, which contain little or no food- 

 yolk, possess only a single membrane 

 formed by the egg itself, and are laid 

 single, an egg-capsule being entirely 

 absent. 



It is always found that such simple 

 eggs are laid in great numbers, for few 

 or none have been rendered sterile, or 

 degraded to serve as the food of 

 others. 



Naturally in such cases the devel- 

 oping organism can attain no great 

 degree of complexity of structure be- 

 fore its original source of food-supply, 

 that contained in the egg itself, is used 

 up. The organism must then hatch 

 out and seek food for itself from extra- 

 neous sources. Under these condi- 

 tions the resulting organism possesses at 

 its birth the simplest possible struc- 

 ture, that of a gastrula composed of two 

 layers of cells and with an aperture 

 leading into the gut. 



Apparently there are two ways in 

 which a further supply of food might 

 be bestowed on the developing egg. 

 Some of its fellows might be rendered 

 sterile in the ovary, and there be used 

 up, as in liver-flukes, tapeworms, in- 

 sects, etc , to increase the food supply 

 of the remaining functional eggs. Or, 



