SEGMENTATION STAGES. 



53 



of cleavage is in the arrangement of the germinal cytoplasm around the nuclei. 

 This is expressed in such a way that the germinal yolk rises between them like a wall. 

 The second stage in which cleavage is seen at the surface is shown in plate iv, 

 fig. 21, from the egg companion to the one shown in plate iv, fig. 20, but incubated 

 longer (about forty minutes). Here a second furrow is noticeable. The 

 resulting " blastomeres " are unequal in size, one of them being as large as two of 



Fig. 48. Section passing between the point I and I of the segmentation stage shown in PI. IV, fig. 20. In the present section the 



line which appeared to indicate first cleavage lies below the point /', below this a vacuole is present whose lateral extent gives one 



the impression of the width of the furrow noted in surface view. X 35. 

 Fig. 49. Section through a segmentation stage corresponding to PI. IV, fig. 22. It will be observed that some of the cleavage lines 



do not open to the surface, as at r. On the other hand, one of the spaces between the blastomeres opens into a fissure-like 



vacuole, a. 

 Fig. 30. Similar section of early segmentation stage, in which, as at >:, a nucleus appears without any neighboring cleavage furrow. 



A line of thicker germinal yolk appears in its place. 

 Fig. 51. Section of early cleavage stage in which, when viewed from the surface, deep, fissure-like vacuoles appear as cleavage lines, 



the others. As in the companion stage the furrows fade away at the margin of the 

 germinal area, and this is again surrounded by a somewhat regular ring of merocyte 

 eminences. It may be mentioned that these characters are materially modified, 

 i. e., as far as surface view is concerned, when the egg is hardened, e. g., in acetic 

 sublimate. And in sections it is found that the circumgerminal fosse and merocyte 

 eminences disappear and what was interpreted as surface furrows in the living egg 

 appear as long and wide vacuoles. Thus in fig. 48, a section transverse to the 



