April 13, 1883.] 



SCIENCE. 



275 



of the nucleus {n) , and extending around the 

 egg as a thin film, has become quite different 

 from the true deutoplasm (d) , which consists 

 of coarse, flattened, ovoidal bodies, as shown 

 in iig. 5 more highlj' magnified. At the upper 

 pole of the egg, and just below the nucleus (n) , 

 the coarser deutoplasm corpuscles or globoids 



rapidlj' become smaller ; so that the nucleus is 

 invested \)y a disk (p) composed of verj' fine 

 granules, which is in a condition intermediate 

 between that of the external protoplasm lawyer 

 (&) and the deutoplasm or j-elk (d) . In verj' 

 immature ova from the same ovar}- of Lepidos- 

 teus, I find the nucleus in a central position, 

 just as in the ovicell of the eel represented" in 

 fig. 1 ; differing, however, in details of its 

 structure, containing, as it does, a distinct 

 network of trabecular fibres. 



The foregoing facts pretty clearly demon- 

 strate the waj' in which the j-elk and germ are 

 differentiated in a meroblastic egg ; namely, 

 by a gradual separation of the germinal and 

 deutoplasmic portions of the ovum, the first 

 becoming concentrated peripherally and at one 

 pole bj' a gradual metamorphosis of the deuto- 

 plasm. As this differentiation takes place, it 

 appears that the germinative vesicle is repelled 

 from its original central position, as shown in 

 figs. 1 and 3, and tliat it never returns to the 

 centre of the deutoplasmic mass, even after the 

 polar cells have been extruded, and the remain- 

 ing portion has been converted into the nucleus 

 of the first cleavage. In this last regard the 

 meroblastic ovum is in most striking contrast 

 with the hololilastic. Mr. E. L. Mark' appre- 

 ciates this when he remarks, " The nucleus 

 appears ultimately to assume a position of 



^ Maturation, fecundation, and segmentation of Limas cara- 

 pestvis. — {Bull. mus. comp. ^oul., vi., N'o. 12, p. 517.) 



equilibrium, not with respect to the whole 

 mass of the egg, but in respect to its active 

 constituents. Is not, then, this peculiarity 

 ultimately, though indirectly, referable to the 

 want of a uniform distribution of deutoplasm, 

 — to the polar concentration of the protoplasm, 

 in other words ? ' ' This covers the ground ; 

 but the writer is inclined to believe that the 

 deutoplasm exercises a veritable repulsive 

 force upon the nucleus, as shown in the egg 

 of Lepidosteus, and that in this wa}' only can 

 we explain the failure of the nucleus to return 

 to the centre of the meroblastic ovum after its 

 metamorphosis attending the expulsion of the 

 polar cells, and the fusion of the male and 

 female pronuclei. 



The displacement of the nucleus due to its 

 migration during the maturation of the egg has 

 a profound influence upon the mode of develop- 

 ment of the various tj^pes, as already urged by 

 Haeckel. A studj' of the mode in which the 

 germ of a fish-ovum is developed may serve 

 to make the nature of this influence clearer. 

 Fig. 2 represents an egg of an osseous flsh in 

 diagrammatic outline, without its membrane, 

 in three phases of maturation, up to the time 

 of germ development, which maj- take place 

 without the influence of the spermatozoon, as 

 shown hj the observations of Ransom, Hoff- 

 mann, and mj'self. In the first phase shown 

 by the figure the protoplasm (p) may surround 

 the deutoplasm (cl), or it may form a scai'cely 

 jDerceptible laj-er on the surface of the egg ; 

 at a later stage this protoplasm has heaped 

 itself up at one pole of the egg, as shown by 

 the line pi; at a still later stage the germinal 

 matter has aggregated itself into a biscuit- 

 shaped germ-mass, the outline of which is 

 shown bj" the dotted line p II. The process 

 is sometimes quite complex, and takes as much 

 as four hours for its completion, as in the cod's 

 egg, in which, as in most fish ova, the disk is 

 formed after tlie emission of the egg from the 

 ovaiy. In other tj-pes a distinct meshwork of 

 protoplasm, continuous with the external layer, 

 is insinuated between large yelk-masses (cZ) , as 

 shown in fig. -1 at t. This arrangement seems 

 to be the typical one amongst clupeoids. The 

 process of germ-development in true osseous 

 fishes is therefore essentially similar to that 

 which we have described as occurring in Lepi- 

 dosteus. 



According to Hoffmann, the nucleus of the 

 first segmentation is not the one nsuallj- hith- 

 erto regarded as such, which is concerned in 

 the division of the germ-disk (p) into two 

 equal blastomeres or cells, but its axis in its 

 spindle stage is placed in a line coinciding with 



