ADDRESS. 19 



one of its poles, and formed at its expense. 9 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 commences at the circumference just within the 

 vitelline membrane, and, extending towards the centre, 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 commencement of development becomes divided into two 

 similar cells. This forms the first stage of cleavage. Bach 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 

 history — has become converted into an organism composed of many 

 thousands of cells. This is one of the most widely distributed phenomena 

 of the organic world. It is called ' the cleavage of the egg,' and con- 

 sists 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 

 acquainted with the further history of the embryo, I would refer to the 

 excellent address delivered two years ago at the Plymouth meeting of 

 the Association by one of my predecessors in this chair — Prof. Allen 

 Thompson. 



That protoplasm, however, may present a phenomenon the reverse of 

 that in which a simple cell becomes multiplied into many, is shown by 

 a phenomenon already referred to — the production of plasmodia in the 

 Myxomycetse by the fusion into one another of cells originally distinct. 



The genus Myriothela will afford another example in which the for- 

 mation of plasmodia becomes introduced into the cycle of development. 



9 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 

 condition assumed by the spindle with any special structure of the quiescent nucleus, 

 it is highly probable that it consists in a rearrangement of fibres already present. 

 That this is really the case is borne out by the observations of Schleicher on the 

 division of cartilage cells. (Die KnorpelaelltJieihmg. Arch, fur Mikr. Anat. Band 

 xvi. Heft 2. 1878.) 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, travelling towards one of its poles, and 

 then towards 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 Streicher 

 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 afterwards 

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



C2 



