THE DIVISION OF THE YOLK. 57 



they might thus be regarded as of sperm-head origin. On the other hand, it 

 might still be claimed that the nuclei of such blastomeres were derived from the 

 segmentation nuclei, for a more careful examination shows (i) that amitoses exist 

 in the blastomeres in the central region of the germ, and (2) that no mitoses are 

 found in the zone of the merocytes, where we may reasonably expect that some 

 nuclei are present which are derived from segmentation nuclei. The problem is, 

 nevertheless, a difficult one, and hardl}- to be answered in the present outline of 

 Chimseroid development. We point out, however, that two criteria which have been 

 given a prominent place in the discussions of shark development can not be 

 emplo3'ed in the present instance, viz. (i) the number of the chromosomes which 

 would naturally give a clue as to the origin of the nuclei can not be estimated in the 

 merocytes, since they are here undergoing only amitotic division; (2) the size of the 

 present merocytes can not prove an important element for comparison, since they 

 range from minute to large, and in shape from spherical to greatly elongated and 

 irregular. 



An important phase of the cleavage in Chima?ra has naturally been introduced 

 by the foregoing discussion, /. c, as to the segmentation of the egg in its extra- 

 germinal region. We have seen that as segmentation progresses nuclei (whether 

 segmentation or sperm-merocyte) spread peripherad. Their presence can be 

 determined in sections ; and in surface view, in the later stages, e. g., plate iv, 

 fig. 28, cleavage lines can be seen passing outward in the region of the circum- 

 germinal zone. That these lines are actual furrows is shown in such a section as 

 that of fig. 56 (at the right). Such marginal furrows, however, are usually minute 

 in size, and are often, in surface view at least, difficult to follow, a difhculty which 

 may be due either to the blending of these delicate lines with the color of the 

 circumgerminal zone, or to the partial or total confluence of the adjacent rims of 

 the cleavage furrows, c. g., as in the marginal blastomeres of Cryptobranchus 

 (according to Ishikawa). The distinctness of these lines, however, increases after 

 the circumgerminal zone is passed, and they later give rise to what we must regard 

 as the most remarkable feature of the segmenting egg. 



To follow this process : In fig. 37 a late stage of segmentation is shown; the 

 germ is z.tg; nearby are nests of blastomeres ((/". fig. 56, ti), two of which are of 

 such size as to appear in the figure, as at^'; from the germ radiate furrows, two of 

 which, a and a , have become conspicuous at the periphery of the egg, where they 

 may have merged with similar furrows, or indeed, on the other hand, there is a 

 possibility that the long furrow passing between the points marked with an asterisk 

 (*) may be the deflected continuation of the lines a and a'. 



In figs. 58 and 59 furrows are seen arising from or near the circumgerminal 

 zone, and examination shows they deepen as they proceed peripherad. In the egg 

 shown in fig. 59 the outgoing line subdivides and marks out superficially a narrow 

 segment of yolk. In the same egg, but in the anterior region, we note another 

 marginal line, b. Examined from below this stage is of considerable interest 



