176 BULLETIN OF THE 



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 of the hen's egg, and will pass to the consideration of the other body, 

 — the vitellus, or egg proper, — to which all the other parts are simply 

 accessories. This vitellus, or yolk, has not more than one twentieth the 

 diameter of the whole egg, and when the latter is freshly laid it appears 

 as a minute speck, 125 /x. in diameter, just visible to the unaided eye 

 as a whitish dot, which usually has an eccentric position. It is to the 

 study of the yolk, and the changes it undergoes, that I shall confine my 

 attention. 



Some of these changes may be followed in the living egg under the 

 microscope ; other and remarkable changes, which up to within a few 

 years had escaped the attention of embryologists, are meanwhile going 

 on within the yolk, and are either altogether hidden, or are only par- 

 tially visible to one studying the living specimen. It is only by the use 

 of certain acid reagents, which have the immediate effect of killing the 

 egg, and at the same time of hardening it, that these internal conditions 

 may be successfully studied. In considering the successive metamor- 

 phoses which the yolk undergoes, it will perhaps be best to follow the 

 course which the observer is compelled to take ; that is, to notice first 

 what may be observed in the living egg, and then to supplement the 

 knowledge thus gained by such instantaneous pictures as the hardening 

 process affords. The more numerous these views, and the more frequent 

 and regular the intervals at which they are taken, the more complete 

 will be the data for interpreting these indirectly observed phenomena. 



The changes which it is proposed to follow in this paper are only such 

 as occur between the time the eggs are excluded and the end of the 

 first segmentation. Inasmuch as the following observations begin with 

 the deposited egg, — i. e. do not include a study of the ovarian egg, nor 

 of any of the changes it undergoes within the body of the parent, — 

 I cannot claim for them the completeness I wish they possessed. 

 Recent studies on the very early stages of eggs of other animals will, 

 however, enable us to make a better use of these limited observations 

 than could be made otherwise. For the time indicated, I trust they 

 will be found tolerably complete and connected. 



The nature of the phenomena which transpire within the limits of the 

 time selected allow one to group the observations about three principal 

 heads : — 



1. The changes connected with the ripening of the egg. 



2. Fecundation of the mature egg. 



3. Segmentation, or cleavage. 



The observations under the first will be least complete, because they 



