August 21, 1879] 



NATURE 



3S9 



It may be generally stated as the results of his observations 

 on the corpuscula of the Coniferse, that the nucleus of the cell 

 about to divide assumes a spindle shape, and at the same time 

 presents a peculiar striated differentiation, as if it were composed 

 of parallel filaments reaching from end to end of the spindle. 

 These filaments become thickened in the middle, and there form 

 by the approximation of the thickened portions a transverse plate 

 of protoplasm (the "nucleus-plate"). This soon splits into 

 two halves, which recede from one another towards the poles of 

 the spindle, travelling in this course along the filaments, which 

 remain continuous from end to end. When arrived near the 

 poles they form there two new nuclei, still connected with one 

 another by the intervening portion of the spindle. 



In the equator of this intervening portion there is now formed 

 in a similar way a second plate of protoplasm (the " cell- 

 plate ''), which, extending to the walls of the dividing cell, 

 cuts the whole protoplasm into two halves, each half containing 

 one of the newly-formed nuclei. This partition plate is at first 

 single, but it soon splits into two lamina:, which become the 

 apposed bounding surfaces of the two protoplasm masses into 

 which the mother cell has been divided. A wall of cellulose is 

 then all at once secerted between them, and the two daughter 

 cells are complete. 



It sometimes happens in the generation of cells that a young 

 brood of cells arises from the parent cell by what is called " free 

 cell formation." In this only a part of the protoplasm of the 

 mother cell is used up in the production of the offspring. It is seen 

 chiefly in the formation of the spores of the lower plants, in the 

 first foundation of the embrj-o in the higher, and in the formation 

 of the endosperm — a cellular mass which serves as the first nutri- 

 ment for the embryo — in the seeds of most Phanerogams. The 

 formation of the endosperm has been carefully studied by Stras- 

 burger in the embyro-:-:ac of the kidney bean, and may serve as 

 an example of the process of free cell formation. The embryo- 

 sac is morphologically a large cell with its protoplasm, nucleus, 

 and cellulose wall, while the endosperm which arises within it 

 is composed of a multitude of minute cells united into a tissue. 

 The formation of the endosperm is preceded by the dissolution 

 and disappearance of the nucleus of the embryo-sac, and then 

 in the midst of the protoplasm of the sac several new nuclei 

 make their appearance. Around each of these as a centre the 

 protoplasm of the mother cell is seen to have become differen- 

 tiated in the form of a clear spherule, and we have thus corre- 

 sponding to each of the new nuclei a young naked cell, which 

 soon secretes over its surface a membrane of cellulose. The 

 new cells, when once formed, multiply by division, press one 

 on the other, and so combining into a cellular mass, constitute 

 the completed endosperm. 



Related to the formation of new cells, whether by division or 

 by free cell formation, is another very interesting phenomenon 

 of living protoplasm known as "rejuvenescence." In this the 

 whole protoplasm of a cell, by a new arrangement of its parts, 

 assumes a new shape and acquires new properties. It then 

 abandons its cellulose chamber, and enters on a new and inde- 

 pendent life in the surrounding medium. 



A good example of this is afforded by the formation of swarm- 

 spores in (.lidogonium, one of the fresh-water algaa. Here the 

 whole of the protoplasm of an adult cell contracts, and by the 

 expulsion of its cell sap changes from a cylindrical to a globular 

 shape. Then one spot becomes clear, and a pencil of vibratile 

 cilia here shows itself. The cellulose wall which had hitherto 

 confined it now becomes ruptured, and the protoplasmic sphere, 

 endowed witli new faculties of development and with powers 

 of active locomotion, escapes as a swarm-spore, which, after 

 enjoying for a time the free life of an animal, comes to rest, 

 and develops it elf into a new plant. 



The beautiful researches which have within the last few years 

 been made by the observers already mentioned, on the division 

 of animal cells, show how close is the agreement between plants 

 and animals in all the leading phenomena of cell division, and 

 afford one m jre proof of the essential unity of the two great 

 organic kingdoms. 



There is one form of cell which, in its relation to the organic 

 world, possesses a significance beyond that of eveiy other, 

 namely, the egg. As already stated, the egg is, wherever it 

 occurs, a typical cell, cinsisting es entially of a globule of pro- 

 toplasm enveloping a nucleus (the " germinal vesicle "), and wi;h 

 one or more nucleoli (the "germinal spots") in the interior of 

 the nucleus. This cell, distinguishable by no tangible charac- 

 ters from thou ands of other cells, is neverllieless destined to 



run through a definite series of developmental changes, 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 — 

 composed, it may be, of countless millions of cells — can be 

 derived from the simple egg cell only by a process of cell multi- 

 plication. 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 under- 

 goes in order to bring it into the condition in which cell multi- 

 plication 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 multiplica- 

 tion of the egg cell by repeated division. I shall further confine 

 myself to an account of this phenomenon as presented in typical 

 cases, leaving out of consideration certain modifications 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 siuTOunding 

 membrane is callled the "vitellary 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 assumej 

 the shape of a spindle, similar to what we have already seen in 

 the cell-division of plants. On each pole of the spindle 

 transparent protoplasm collects, forming hece a clear spherical 

 area. 



At this time a very striking and characteristic phenomenon is 

 witnessed in the egg. Each pole of the spindle has become the 

 centre of a system of rays which stream out in all directions into 

 the surrounding protoplasm. The protoplasm thus shows, 

 enveloped in its mass, two sun-like figures, whose centres are 

 connected to 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," suggested by its connec- 

 tion 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, and each half 

 recedes from the equator and travels along the filament towards 

 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 

 gradually drau n into the substance of the two globular masses, 

 finally disappears. 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 



* 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 speci.al 

 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 

 nut by the observations of Schleicher on the division uf cartilage cells. (" Die 

 Knorpelzclltheihing," Arch, fiir 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 nuclcu-s, now 

 deprived of its membrane, may wander about the cell, travelling tow.-iru» one 

 of Its pules, and then towards the other; or it may at one time contract, and 

 then again dil.ate, to such an extent as nearly to fill the entire cell. To this 

 nuclear activity Schleicher applic:i the term " Kai->'okinesis.'* It results in a 

 nearly parallel arrangement of the nuclear filaments. Then these converge 

 at theT extremities .and become more widely separated in ihe middle, so as 

 to give tj the nucleus the form of a spindle. The filaments then iiecome 

 fused together at each pole of the spindle, so as 1 1 form the two nc« 

 nuclei, which are at first nearly homogeneous, but which afterwards beconM 

 broken up into their component filaments, rodit, and granules. 



