738 THE POPULAR SCIENCE MONTHLY. 



and each half recedes from the equator and travels along the filament 

 toward its extremity. When arrived at the poles of the spindle each 

 set of half knots becomes fused together into a globular body, while 

 the intervening portion of the spindle, becoming torn up, and gradu- 

 ally drawn into the substance of the two globular masses, finally dis- 

 appears. And now, instead of the single fusiform nucleus, whose 

 changes we have been tracing, we have two new globular nuclei, each 

 occupying the place of one of its poles, and formed at its expense.* 

 The egg now begins to divide along a plane at right angles to a line 

 connecting the two nuclei. The division takes place without the 

 formation of a cell-plate such as we saw in the division of the plant- 

 cell, and is introduced by a constriction of its protoplasm, which com- 

 mences at the circumference just within the vitelline membrane, and 

 extending toward the center, divides the whole mass of protoplasm 

 into two halves, each including within it one of the new nuclei. Thus 

 the simple cell which constituted the condition of the egg at the com- 

 mencement of development becomes divided into two similar cells. 

 This forms the first stage of cleavage. Each of these two young cells 

 divides in its turn in a direction at right angles to the first division- 

 plane, while by continued repetition of the same act the whole of the 

 protoplasm or yolk becomes broken up into a vast, multitude of cells, 

 and the unicellular organism the egg, with which we began our his- 

 tory has become converted into an organism composed of many 

 thousands of cells. This is one of the most widely distributed phe- 

 nomena of the organic world. It is called " the cleavage of the egg," 

 and consists essentially in the production, by division, of successive 

 broods of cells from a single ancestral cell the egg. 



It is no part of my purpose to carry on the phenomena of develop- 

 ment further than this. Such of my hearers as may desire to become 



* Though none of the above-mentioned observers, to whom we owe our knowledge of 

 the phenomena here described, seem to have thought of connecting the fibrous condi- 

 tion assumed by the spindle with any special structure of the quiescent nucleus, it is 

 highly probable that it consists in a rearrangement of fibers already present. That this 

 is really the case is borne out by the observations of Schleicher on the division of carti- 

 lage-cells. (" Die Knorpelzelltheilung," " Arch, fiir mikr. Anat.," Band xvi., Heft 2, 

 18*78.) From these it would appear that, in the division of cartilage-cells, the investing 

 membrane of the nucleus first becomes torn up, and then the filaments, rodlets, and 

 granules, which, according to him, form its body, enter into a state of intense motor 

 activity, and may be seen arranging themselves into star-like, or wreath-like, or irregular 

 figures, while the whole nucleus, now deprived of its membrane, may wander about the 

 cell, traveling toward one of its poles, and then toward the other ; or it may at one time 

 contract, and then again dilate, to such an extent as nearly to fill the entire cell. To 

 this nuclear activity Schleicher applies the term " Karyokinesis." It results in a nearly 

 parallel arrangement of the nuclear filaments. Then these converge at their extremities 

 and become more widely separated in the middle, so as to give to the nucleus the form 

 of a spindle. The filaments then become fused together at each pole of the spindle, so 

 as to form the two new nuclei, which are at first nearly homogeneous, but which after- 

 ward become broken up into their component filaments, rods, and granules. 



