LECTURES ON EMBRYOLOGY. 



81 



side four smaller ones. We have still in another, 

 less regularity. Four less spheres are formed, and 

 between them two large ones, and two very small 

 ones; and soon, hy multiply ing the divisions, we 

 arrive finally at the state of the yolk, when it is 

 composed of a mass consisting of many yolk cells, 

 in each of which there is a clear sphere, As there 

 is one forming in each division when the process 

 of dividing the yolk has only divided the mass into 

 fewer spheres. 



About the time when the whole mass is reduced 

 into small cells, there are vibrating Cilia coming 

 out from the surface of some of these eggs (Plate 

 XL, figs. A, B, C, D.) 



But the most curious phenomenon which takes 

 place is this : that the whole yolk does not con- 

 stantly go on to form one single individual, But 

 there may be instances when the mass of yolk 

 which has been subdivided into cells, is itself divi 

 ded into two, or three or more masses, which grow 

 independently, several individual animals arising 

 from one yolk; several individual animals aris- 

 ing from one mass of yolk, which thus divides. 

 And in this process of the division of a whole mass 

 into several individuals, there are isolated cells, 

 which are separated from the main mass, and 

 continue to live and to rotate by the agency 

 of their vibratile Cilia with the main mass. And 

 in such a case we have the wonderful sight of two 

 or more germs, having been derived from the di- 

 vision of one unique mass of yolk, constituting 

 two or three, or more individuals, each moving for 

 itself and rotating with the others in one yolk 

 membrane, and isolated cells which also rotate be- 

 tween. So that individual loose cells maintain for 

 a time a separate life, and continue to live during 

 the whole period of growth of the larger animals 

 within the egg membrane; and those isolated, scat- 

 tered cells die only when the larger germs, which 

 will grow into perfect animals, have been hatched, 

 or pressed out from the vitelline membrane. 



Nothing could show more distinctly that there 

 is independence of life in the cell than the fact of 

 this isolation. But what the combining power is 

 between those cells which grow and form individ- 

 ual animals, can scarcely be understood under 

 such condi ions. Whence the action of the vi- 



t 



tal principle which keeps the cells together, oiigi- 

 nates, escapes our intelligence. Indeed, nothing is 

 more astonishing than to see that under slight 

 pressure, such a germ may be resolved into loose 

 cells, whose Cilia will continue for a short time to 

 vibrare,in the same manner as a nebular mass seen 

 through a powerful telescope may be resolved into 

 individual stars, which nevertheless form a pecu- 

 liar cluster of isolated bodies ; similar to the cells 

 with individual life, which constitute, as it were, 

 similar clusters. And when they have gone beyond 

 this period of life, then they have undergone a 

 more intimate connection, which prevents their dis- 

 solving again ; and then they go on constituting a 

 new being. Then during the further changes, by 



which they now assume the form of the parent 

 animal, there are constantly isolated cells cast from 

 the main body, which revolve for a short time, and 

 then die, This process, which is exemplified here 

 In the early condition of life, and under a simple 

 condition of structure, is well known to take place 

 in many animals, which east their skin repeatedly 

 during life, as the caterpillar ; or Mollusks, which 

 cast their external coating under the form of mu- 

 cus; or other animals, which cast their hairs; or in 

 our own body when the epidermis is cast and oth- 

 er cells are formed to take the place of those which 

 fall off in the form of small scales. So that you 

 see the remarkable phenomenon of the isolated 

 cells of Eolis, is only what we have on a still great- 

 er scale in higher animals, where millions and mil- 

 lions of cells are constantly east from the surface 

 of fuil grown individuals. 



These cells consist permanently and uniformly of 

 an external envelope, a thin membrane containing 

 a fluid, within which there is another vesicle called 

 the nucleus, and in the centre of which,there is still 

 another called the nucleolus,so that a perfect cell in 

 its perfect condition is a sphere enclosing two other 

 spheres, the innermost one being the smallest, ap- 

 pearing like a granule. In such cells as are represen 

 ted in Plate XXXVII, we have figures with which 

 we have been familiar from other illustrations. A 

 cell in its perfect condition has the same structure 

 as an egg in its primitive formation. Here we ar- 

 rive at a most unexpected, but universal, uniform 

 structure, not only of cells, but of the primitive 

 substance of wh'ch new individuals are to be form- 

 ed. What we call eggs in their simple condition, 

 are cells of a peculiar structure, formed in a pecu- 

 liar part of the body, destined to undergo peculiar 

 modifications, by which the body is not enlarged, 

 by which no particular function is performed, but 

 by which a new individual is formed. So that in 

 every point of view we find unity in the structure 

 of animals, even in the structure, compared with 

 the mode of re-production ; the cells of which the 

 tissues consist being identical in structure with the 

 eggs by which new individuals are produced. 



There is a question which may be asked, and to 

 which I hope to give at least a partial answer. 

 How are these cells formed? and how are these 

 eggs formed? We have examined the mode of 

 formation of the germs. Let us now examine the 

 mode of formation of the eggs. 



I have been fortunate enough to trace them 

 through all their phases of formation in Mollusks, 

 and I think there has not been a link in their trans- 

 formation which has escaped my attention. So 

 that the whole process of their multiplication has 

 been directly observed. Tracing the formation of 

 eggs will be tracing the formation of cells, the mo- 

 ment it is understood that cells and eggs have the 

 same structure. When examining very young ova- 

 riesfor we must not take the egg when laid we 

 must not take them when formed within the ovary 

 we must not take even a full grown ovary 



