CHIM^ROID FISHES AND THEIR DEVELOPMENT. 

 54 



first cleavage line of the stage of plate iv, fig. 20, the fosse will be seen to have dis- 

 appeared, and the line which indicated cleavage, so conspicuous in surface view, 

 now appears under the point / as a vacuole triangular in section, its apex touching the 

 surface of a germ.* Vacuoles, we note, are abundant in early stages; several 

 are present in the section of the germ just referred to, and from their arrange- 

 ment they suggest the division of the germ into blastomeres. 



Three stages of earl}^ cleavage are shown in surface view in plate iv, figs. 22, 

 23, and 24, the first as an opaque object, drawn from the living egg, the second and 

 third as translucent objects, in the last cases the germ having been removed, jilaced 

 in a watch glass, and examined by transmitted light during the process of fixation. 

 In these three preparations there is considerable irregularity in the surface charac- 

 ters; in the first the margins of the blastomeres are rounded, in the others angular; 

 outwardh' they appear to represent fourth and fifth cleavages; in section, how- 

 ever, single "blastomeres" are sometimes found to contain several segmentation 

 nuclei. It was observed that the resting and dividing nuclei were sometimes found 

 in the same section, and it follows accordingly that in Chima?ra the s^mchrony of 

 cleavage is earh' lost. 



Four later stages of segmentation appear in plate iv, figs. 25, 26, 27, and 28, all 

 drawn under conditions of transmitted light, the living specimens having been 

 removed and examined in watch glasses. In the first of these the germ is well 

 marked off from its circumgerminal zone; in the rest some of the marginal cleav- 

 age lines were traced half-way across the circumgerminal zone, and in a few 

 instances these lines could be followed quite across it. In these stages continued 

 subdivision of the "blastomeres" has taken place, those in the central position 

 becoming divided oftener than those near the peripher}-. As in earlier stages, some 

 of the cleavage lines are probabl}' not expressed at the surface, and are due only to 

 vacuoles; the latter are lineal in surface view, sometimes wide, sometimes narrow, 

 occasionally almost attaining the surface, at other times lying fairly deep in the 

 germ. (C/lfigs. 49, 50,51.) Sometimes, as in fig. 49, they are actuallv continuous 

 with cleavage furrows, as at a, and considering the relation which the}' often bear 

 to nuclei {e.g., iitfra, under the heading "gastrulation"), we conclude that in 

 some cases — even, indeed, in many cases — they are homologous to cleavage spaces, 

 /. c, that the}' are cleavage spaces which fail to become expressed at the surface 

 of the germ. This conception appears to be applicable even when the vacuoles 

 appear in the peripheral region of the germ in fertilization stages. Thus in fig. 34 

 the masses of germinal yolk separated by the vacuoles (under the points marked 

 with an asterisk [''']) usually bear sperm nuclei which, as we know by analogy, will 

 cause ' ' segmentation. " Accordingly', even in this position vacuoles may be compared 

 to intercellular spaces, at least from the standpoint of developmental mechanics. 



In fig. 52 a section of a segmentation stage corresponding to plate iv, fig. 26, 

 shows that cleavage has by this time extended deep into the germinal area. Hori- 

 zontal divisions have occurred, irregularis' however, for in some places the, blastoderm 



*There is thus a possibility of there having been an open furrow in the living egg. 



