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E. A. SCHAFER. 
should on no account be lost sight of^ yet it is so manifestly 
convenient for purposes of description and of comparison to 
maintain the idea of grades, that I shall here adopt a like simile, 
choosing a succession of telescopic tubes, instead of a staircase, 
to represent the succession of developmental phases, because 
those can be imagined to be more or less drawn out, and shifted 
upon one another, so as to illustrate the varying relations which 
the typical successive phases bear to one another in the develop- 
ment of different animals. It will further be necessary to imagine 
the tubes constituted of some very elastic material, rendering it 
possible for them to be extended to two, three, or many times 
their proper length, or, on the other hand, to become shortened 
so much as to be scarcely perceptible. Each tube of which the 
telescope is composed may be taken to represent a particular 
stage of development, and the greater the complexity of the 
organism the more tubes must be added to the series, and the 
greater the length of the developmental telescope. 
If we now proceed to compare the development of various 
animals with one another, we are struck with the resemblance — 
with the identity I might rather say — of the first stages in all. 
To begin with all take origin in an ovum ; and although at the 
first glance this appears so different in many (compare the ovum 
of a mammal with the egg-yolk or pvum of the bird), exami- 
nation shows it to be in all cases similar in essential composition. 
For the difference depends solely upon the amount of nutritive 
material which has been stored up within and in connection with 
the reproductive protoplasm, and has relation to the greater or 
less facility with which a supply of food can be obtained from 
without during the active developmental process. To use a fa- 
miliar comparison, the difference is precisely the same as that 
between an army operating in a fertile country where supplies are 
readily obtainable, and a force which is to operate in a region 
barren or devastated, and far removed from the base of supplies. 
In the one case pabulum sufficient merely for immediate use is 
required, in the other a great store of nutriment must be accu- 
mulated and must accompany and proportionately hamper every 
movement of the army. Moreover, since to a part of the force 
must be entrusted the duty of guarding the store and distributing 
it as required, a greater number of men is required to bring the 
army up to the same proportionate strength as that of the unen- 
cumbered force. So in those ova which are provided with a 
large store of nutritive material, do we find that when the active 
movements which characterse development commence, those 
movements are hampered by the presence of the inert food-ma- 
terial, and it is precisely as the store becomes lessened that the 
apparent difference between such ova as these, and those others 
