August 1 6, 1877] 



NA TURE 



■,07 



cleavage or splitting up of the protoplasmic substance of the 

 yolk, by which it becomes rapidly subdivided into smaller and 

 mere numerous elements, so as at last to give rise to the pro- 

 duction of an organised stratum of cells out of which by subse- 

 quent changes the embryo is formed. 



The process of voIIl segmentation may at once be distinguished 

 as of two kinds, according as it affects in the small-yolked ova 

 the whole mass of the yolk simultaneously, or in the large-yolked 

 ova is limitfd to only one part of it. The cleavage process, in 

 fact, affects the geiminal and not the food yolk ; S3 that to take 

 the two most contrasting instances of the bird and mammal to 

 which I have before referred, it appears that while the mammal's 

 ovum undergoes entiie segmentation, this process is confined to 

 the substance of the cicatricula or geiminal disc of the bird's 

 egg. This process is essentially one of cell-division, but it is 

 also in some measure one of cell-formation. The best idea of its 

 nature will be obtained ftom a .=hort description of the total 

 segmentation occurring in the mammal's ovum. 



When, as before mentioned, the germinal vesicle has been in 

 part extruded or lost to sight, the whole yolk-substance of the 

 ovum forms a nearly uniform mass of finely granular proto- 

 plasm, inclosed within the external cell-membrane. Within a 

 few houis later a clear nucleus has aiisen in this mass. To this 

 more definite form of organisation, assumed by the germinal 

 substance of the future animal which is about to be the subject 

 of the segmenting process, the name of the first segment- 

 sphere may be given. 



By the process of clea\ age, which now begins, this first seg- 

 ment-sphere and its nucleus undergo division into two nucleated 

 spheres of smaller size, the whole substance of the yolk, in a 

 holoblastic ovum, such as that of the mammal, being involved 

 in the segmenting proces?. 



The secord stage of division follows after the lapse of a few 

 hours, and results in the formation of four nucleated segment 

 spheres ; and the process of division being repeated in a certain 

 definite order, there result in the succeeding stages, that is, the 

 third, fourth, fifth, and up to the tenth, the numbers of 8, 12, 

 16, 24, 32, 48, 64, and 96 nucUated yolk-spheres, germ-spheres, 

 or formative cell."^. 



In the rabbit's ovum the tenth stage is reached in less than 

 three daj s ; and as during that time the size of the whole ovum 

 has undergone very little increase, it follows that the spheres uf 

 each succeeding set, as ihey become more numerous, have dimi- 

 nished greatly in size. These segment-spheres are all destitute 

 of external membrane, but are distinctly nucleated ; and their 

 protoplasmic substance is moie or less gianular, presenting the 

 usual histological characters of growing cells. 



By the time that segmentation has reached the seventh or 

 eighth stage, when 32 or 48 spheres have been formed, the ovum 

 has assumed the appearance of a mulberry, in which the outer 

 smaller spheres, closely massed together, project slightly and 

 uniformly over the whole surface ; while the interior of the ball 

 is filled vvith cells of a somewhat larger size and a more opaque 

 granular aspect, also resulting from the process of segmentation. 



Already, however, the mutual compre-sion of the spheres or 

 cells on the surface, by their crowding together, has led to the 

 llatteniiig of their adjacent sides ; and by the time the tenth 

 stage is reached, when the whcle number of the cells is about 

 96, the more advanced superficial cells having ranged them- 

 selves closely together, form a nucleated cellular layer or 

 covering of the jolk, inclosing within tliem the larger and more 

 opaque cells, derived like the first .'rom the segmenting process. 

 In a more advanced stage, the deeper cells now referred to 

 having also taken the form of a lajer, there results at last the 

 bilaminar blastoderm or embryonic j^erminal membrane. 



The process of [.arlial segmentation, such as occurs in the 

 bird's egg, though perhaps fundamentally the same as that of 

 the mammal previously described, stands in a different relation 

 to the parts of the whole yolk or egg, and consequently differs 

 in its general phenomena. The segmentation is mainly re- 

 stricted in the mcroblastic ova of birds to the germinal disc or 

 cicatricula, and does not immediately involve any part of the 

 larger remainder of the yolk. This takes place during the time 

 of the descent of the yolk through the oviduct, when the yolk 

 is receiving the covering of the white or albumen, the membrane 

 and the shell, previous to being laid — a progress which, in the 

 common dom.est ic fowl, usually occupies less than twenty-four 

 hours. Coiresprmding essentially to the more complete segni'.n- 

 tation of the niammal's ovum, the process leads to the same 

 result in the production of two layers of nucleated formative 

 cells in the original seat of a protoplasmic disc ; a bilaminar 



blastoderm resulting as in the mammal's ovum, though in a 

 somewhat different relation to the yolk. 



I wiil not fatigue you with a description of the details of these 

 phenomena, interesting as they may be, but only mention gene- 

 rally that they consist in the formation of deep fissures running 

 from the surface into the substance of the germ-disc. The first 

 of these fissures crosses the disc in a determin.ate direction, 

 dividing it into two nearly equal semicircular parts. In the next 

 stage another fissure, crossing the first nearly at right angles;, 

 produces four angular segments. Then come four intervening 

 radial fissures, which subdivide the four segments into eight • 

 and next afterwards the central angles of these eight radial seg- 

 ments are cut off from their peripheral portions by a different 

 fissure, which may be compared to one of the parallels of lati- 

 tude on the globe near the pole where the radial or hiivi/iaii 

 fissures converge. And so thereafter, by the succession and 

 alternation of radial and circular clefts, which, however, as they 

 extend outwards, come soon to lose their regularity, the whole 

 germinal disc is divided into the two layers of nucleated cells, 

 constituting the blastoderma or germinal membrane of Pander 

 and all subsequent embryologists.' If a laid egg be sub- 

 jected to the heat of incubation for eight or ten hours, the cica- 

 tricula, now converted into this segmented blastoderm, is found 

 to be considerably expanded by a rapid multiplication of its 

 constituent cells, and in as many more hours, by further changes 

 in its substance, the first lineaments of the chick begin to make 

 their appearance. Similar changes affect the blastoderm of the 

 mammal, and thus it appears that the result of segmentation, 

 in the bird as well as in the mammal and other animals, is the 

 production of an organised laminar substratum, which is the 

 seat of the subsequent embryoric development. 



I must still request your attention to some details connected 

 with the process of segmentation, which bear upon the question 

 of the origin of the new cells, and on which recent research has 

 thrown a new and unexpected light. 



With respect to the nature of the first segment-sphe;e of the 

 ovum and the source of its nucleus, as well as of the other 

 segment-spheres or cells which follow each other in the successive 

 steps of germ-su'odivision, it appears piobable from the re- 

 searches of several independent observers, and more especially 

 of Edward Van Beneden and Oscar Hertwig, that in the course 

 of the extrusion of the germinal vesicle, a small portion of it 

 remains behind in the form of a minute mass of hyaline sul stance, 

 to which Van Beneden has given the name of promtclms, and 

 that, as the result of the fertilising process, there is formed a 

 second similar hyaline globule or pronucleus, situated near the 

 surface, which gradually travels towards the centre and unites 

 with the first pronucleus, and that these two pronuclei, being 

 fused together, form the true nucleus of the first segment sphere. 

 According to this view the original germinal vesicle, when it dis- 

 appears, or is lost to sight, as described by so many embryo- 

 logists, is not dissipated, but only undergoes changes leading to 

 the formation of the new and more highly endowed nucleus of 

 the first embryonic or segmental sphere. It further appears that 

 the sub-division of each segmenting mass is preceded by a change 

 and division of the nucleus, and that this division of the nucleus 

 is accompanied by the peculiar phenomenon of a double conical 

 or spindle-shaped radial lineation of the protoplasm, which, if 

 we were inclined to speculate as to its nature, seems almost as if 

 it n}arked out the lii es of molecular force acting in the organising 

 process. These lines, however, it will be understood, if visible 

 with the microscope, even of the highest magnifying power yet 

 attained, bekng to much larger panicles than those of the sup- 

 posed molecules of the physicist ; but considered in connection 

 with what we know of the movements which frtquently precede 

 the act of division of the yolk-spheres, we seem in this pheno- 

 menon to have made some near approach to the observation of 

 the direction in which the molecular forces operating in organisa- 

 tion may be supposed to act.'- 



' The more exict nature of the process of segmentation was first made 

 known by the interesting researches of liagge in 1841, and more especially 

 of Kolliker in 1S43. The phenomena of complete segmentation were first 

 fully described in the mammal's ovum in Eischoif 's " Description of the De- 

 velopment of the Rabbit," 1842, and followed out in hissucceedmg " Memoirs 

 on the Dog, Guinea-pig, and Roe-deer." The pheoomena of partial segmen- 

 tation were first made known, in their more exact form, by Kblliker's " Re- 

 searches on ihe Development of the Cephalopoda," published in 1S44. In 

 birds the process was lirst described by Uergmann in 1846. and more fully 

 by Costem 1840. 



" The observations refened to above as to ihe division of the nucleus are 

 so novel and of such deep interest that I .-^m tempted 10 add here a short 

 abstract of their more important results from a very clear account given of 

 them by Dr. John PriesUey, of Owens College, Manchtsler, in ihe Journal 

 0/ Microsco/'icat Science for April, 1876. 



