MEROCYTES AND BLASTODERM. 85 



distinct ventral line of boundary, broken only at points, as at b and b, where cells 

 from the yolk are entering. There can be no question in this regard since the 

 entering cells are distinguishable as large in size, circular in outline, and granular 

 in content. (Cf. sections L, M.) Another noteworthy feature in this section is that 

 some of the ectoderm cells as at c and c, give off amoeboid processes and, I am 

 led to believe, later become detached, contributing to the growth of the mesoblast. 

 A detail of this condition is shown in section j. We may finally note that the 

 body cavity, be, reaches its maximum size in this region of the embryo. 



In G the floor of the gut becomes cellular ; the notochord is again separate from 

 the gut wall ; and as before merocytes contribute directly to the growth of the yolk 

 entoderm. In n the last-mentioned character is seen even to better advantage, 

 for not only are the large yolk-cells passed into the lateral yolk entoderm, but they 

 appear also high up in the central gut wall, as atg, and in the region of the peristo- 

 mial mesoblast, as at m. 



In i, finally, a section is shown passing through the region of the head tip, 

 which now projects forward above the blastoderm. On either side of the gut the 

 mesoblast is distinct, differing in this regard from the condition shown in an 

 elasmobranch (cf. Ziegler's figure 19, n, Arch. f. mikr. Anat., Bd. xxxix, Taf. iv). 

 In the neighboring blastoderm, as in the shark, the mesoblast is limited to a small 

 tongue of peristomial cells. 



Before concluding an account of this stage two of its features still deserve 

 comment, (i) The fissuring of the yolk region. The fissures are usually vertical, 

 as indicated in all the foregoing sections, and may, as we have already seen, be 

 regarded as homologous with cleavage spaces. (2) The mode by which merocytes 

 become cells of the embryo. This heading, however, deserves to be treated in a 

 more formal way. 



THE TRANSFORMATION OF MEROCYTES INTO CELLS OF THE BLASTODERM. 



In this connection a number of details of sections of stage D have been figured, 

 figs. 71 K-II, and in examining the series we find evidence, first of all, that 

 merocytes move from a lower into a higher zone of the yolk. Thus, in fig. 7 1 o, the 

 merocytes are elongated in the direction of the yolk-entoderm. * Also in the 

 three sections L, M, and N we observe a great yolk cell (megasphere)f first deep in 



*That this is connected with a migration of these elements in the direction of the surface of the cell mass is 

 known by analogy witness the behavior of slime cells in the skin of amphibians and fishes (e. g., Homea). 



fThe megaspheres can have little to do with primitive ova, since they occur widely scattered throughout the 

 blastoderm. Thus in fig. 71 H one is arising at the extreme rim of the blastoderm, in F several are seen midway 

 between the embryo and the rim of the blastoderm, in H one occurs near the middle of the floor of the gut; others 

 appear in mesoderm and others still in ectoderm. In these several regions they are seen to undergo division, losing 

 more and more of their appearance as megaspheres (v. fig. 71 P). It can not be believed, therefore, that these elements 

 are to be regarded as primitive ova, destined to carry the segregated germ plasm into the embryonic genital folds, for 

 this would involve a conception of primitive ova traveling about extravagantly, from the gut wall to the rim of the blas- 

 toderm, a conception the more improbable when we consider that the urogenital region, to which primitive eggs 

 naturally belong, is already indicated by this stage, as at , fig. 71 G. On the other hand, it follows, I believe, that 

 the evidence provided by Chimsera strengthens materially the position of Ruckert that the megaspheres in elasmo- 

 branchs are to be regarded not as primitive ova but as highly specialized bearers of nutriment, capable of carrying 

 into the midst of embryonic tissues centers of new formative energy. These as single large cells could be passed 

 through the intervening tissue more effectively than could the many small cells to which they give rise, for the resistance 

 of an embryonic tissue to the penetration of cells is obviously proportioned to the surface-contact of the invading cells. 



