PROTOPLASM AND LIFE. 737 



which have as their end the building up of an organism like that to 

 which the egg owes its origin. 



It is obvious that such complex organisms as thus result com- 

 posed, it may be, of countless millions of cells can be derived from 

 the simple egg-cell only by a process of cell-multiplication. The 

 birth of new cells derived from the primary cell or egg thus lies as 

 the basis of embryonic development. It is here that the phenomena 

 of cell-multiplication in the animal kingdom can in general be most 

 satisfactorily observed, and the greater number of recent researches 

 into the nature of these phenomena, have found their most fertile field 

 in the early periods of the development of the egg. 



A discussion of the still earlier changes which the egg undergoes 

 in order to bring it into the condition in which cell-multiplication 

 may be possible, would, however full of interest, be here out of place ; 

 and I shall therefore confine myself to the first moments of actual 

 development to what is called " the cleavage of the egg " which is 

 nothing more than a multiplication of the egg-cell by repeated divi- 

 sion. I shall further confine myself to an account of this phenomenon 

 as presented in typical cases, leaving out of consideration certain modi- 

 fications which would only complicate and obscure our picture. 



The egg, notwithstanding the preliminary changes to which I have 

 alluded, is still at the commencement of development a true cell. It 

 has its protoplasm and its nucleus, and it is, as a rule, enveloped in a 

 delicate membrane. The protoplasm forms what is known as the 

 vitellus, or yolk, and the surrounding membrane is called the " vitel- 

 lary membrane." The division which is now about to take place in it 

 is introduced by a change of form in the nucleus. This becomes 

 elongated, and assumes the shape of a spindle, similar to what we have 

 already seen in the cell-division of plants. On each pole of the spin- 

 dle transparent protoplasm collects, forming here a clear spherical area. 



At this time a very striking and characteristic phenomenon is wit- 

 nessed in the egg. Each pole of the spindle has become the center of 

 a system of rays which stream out in all directions into the surround- 

 ing protoplasm. The protoplasm thus shows, enveloped in its mass, 

 two sun-like figures, whose centers are connected with one another by 

 the spindle-shaped nucleus. To this, with the sun-like rays streaming 

 from its poles, Auerbach gives the name of " Karyolitic figure," sug- 

 gested by its connection with the breaking up of the original nucleus, 

 to which our attention must next be directed. 



A phenomenon similar to one we have already seen in cell-division 

 among plants now shows itself. The nucleus becomes broken up into 

 a number of filaments, which lie together in a bundle, each filament 

 stretching from pole to pole of the spindle. Exactly in its central 

 point every filament shows a knot-like enlargement, and from the close 

 approximation of the knots there results a thick zone of protoplasm in 

 the equator of the spindle. Each knot soon divides into two halves, 



VOL. XT. 47 



