CLEAVAGE STAGES. 



55 



varies in thickness from one to five cells. Noteworthy is the irregularity of the yolk 

 wall out of which blastomeres are segmenting, as at x, and into which deep inter- 

 cellular spaces are continued, becoming confluent below, as at v, v, v, with vacu- 

 oles like those described in earlier stages. It may be remarked that in this stage 

 the vacuoles pass deeply into the yolk. 



Another stage of late segmentation (or blastula), corresponding in general with 

 plate iv, fig. 27, is shown in the series of sections, figs. 53-56. In the section, fig. 53, 

 which passes near the center of the germ, the conditions differ little from the 

 preceding stage. We observe that continued divisions have taken place and that 

 there is still the same outcropping of blastomeres from the yolk wall, as at .a: and x, 

 following mitoses. An advancing character in this stage is the general flattening of 

 the germinal wall, as at the point y, a preliminary step toward the formation of the 

 floor of the segmentation cavity, and possibly indicating fore and aft differentiation 

 of the germ. It may be remarked that this is the first stage in which a conspicuous 

 zone of merocytes was seen. These are numerous under the central blastomeres, 

 most numerous under the peripheral blastomeres, and then rapidly decrease in 





^V. :;;: 



Fig. 52. Section of late cleavage stage (corresponding to PI. IV, fig. 26). x. Mass of germinal yolk from which a 

 blastomere is being budded out ; ?', vacuoles which are continuous with intercellular spaces. 



number peripherad. The three sections, figs. 54-56, illustrate such a series. The 

 first of them, fig. 54, indicates the relation of the above-mentioned vacuoles to 

 intercellular spaces, as at the points marked with an asterisk (*) ; the second and 

 third, figs. 55, 56, are instructive as showing the extension of a nest of cells, n 

 (it is the same group in both sections it appears, however, at the left in the lower 

 section, since this has been turned over on the slide), beyond the margin of the 

 circular mass of blastomeres instructive, since it suggests that the outlying 

 region of the germ (circumgerminal zone) is still little different from the germ 

 itself in its cell-forming nature. It is also to be observed that deep fissures 

 representing intercellular spaces (fig. 56, 2), extend peripherad through the 

 germinal yolk, corresponding to the marginal furrows described in the eggs of 

 ganoids and dipnoi, of Heterodontus, even of amphibia. There is here accord- 

 ingly a region in which, side by side, occur small blastomeres, large yolk masses 

 (bearing nuclei), and undivided yolk ; there is no gradual transition from the yolk 

 to the large blastomeres and from these in turn to the small ones, which, as 

 we have seen, correspond in size with blastomeres of the center of the germinal 

 area. We observe, furthermore, that the small blastomeres arise in any 



