64 



CHIMi^ROID FISHES AND THEIR DEVELOPMENT. 



Continuing our examination of the present specimen, it will be observed that 

 the growth of the cell mass is taking place at both anterior and posterior margins. 

 Cells are still being contributed to the cellular wall behind the archenteron, judging 

 at least from their relations to the yolk — such a blastomere, for example, as that 

 near 2 having become detached from the germinal wall. And at the extreme 

 anterior region of the blastoderm many cells are being budded out of the germinal 

 wall. Thus, as shown in the detail (fig. 63 a), it will be seen that in an overhanging 

 portion of the germinal wall, as below and between the points marked with aster- 

 isks (*)' 3- row of half a dozen cells are clearly outlined in the wall — a condition which 

 indicates strongly a similar origin for the adjacent cells. At lower points of the 



63 ,, ___^^..^__^ /^, 



i^ 





Fig. 63. — Sagillal section of earliest gastrula. ii, Archenteric cavity. !'/>, Blastopore, ii , Segmentation cavity. I V indicate 



position of nuclei in yolk region. 

 Figs. 63 A and B. — Details of foregoing section at anteriormost and posteriormost margins respectively. 



germinal wall, finally, cellular additions to the blastoderm are being made. Note- 

 worthy in the present section are the vacuoles which pass deeply into the yolk and 

 suggest, as we have already noted, modified or suppressed lines of cleavage; espe- 

 cially well marked are those occurring in the fine yolk on either side of the blasto- 

 derm, since they form a series of vertical fissures and mark off masses of fine 3'olk 

 containing nuclei. The vacuoles also occur throughout the neighboring coarse yolk, 

 and in connection with their appearance there we note the presence of merocytes 

 which have traveled, as at iv, iii, 11 or i, far out over the yolk. We note, lastly, 

 the way in which the fine yolk passes down in rifts into the coarse yolk, for this 

 suggests again the modified holoblastic condition of the egg. 



The next stages in gastrulation deal with the extension of the blastoderm over 

 the yolk. Thus in fig. 64 is given a sagittal section of a stage in which the diameter 



