LATER GASTRUL/E. . 75 



LATER GASTRUL^.. 



Surface views of three later o^strula; are pictured in plate v, figs. 33-35, a 

 series in which the body of the embryo becomes distinctly differentiated. In the 

 first it occurs as a lip-like thickening, the blastoderm itself having become some- 

 what larger in diameter and flatter than in the previous stage. In the present 

 specimen, which was examined after my interest was aroused in the matter of the 

 peripherad migration of the yolk-nuclei, these structures could be seen* spread out 

 widely over the neighboring surface of the yolk. The second stage, plate v, fig. 34, 

 resembles outwardly a shark embryo at Balfour's stage b; the light area in the 

 anterior and median portion of the blastoderm, which marks the cleavage cavity, is, 

 however, larger than in any selachian hitherto described. In the third stage, plate v, 

 fig. 35, the embryo arises as a knob-like eminence, its tail end projecting some- 

 what over the edge of the blastoderm; anteriorly the surface of the blastoderm 

 becomes thin and transparent, and it here assumes a peculiar vesicular character. 



Details of the Later Gastrula of Plate V, Fig. 35. 



This stage, although scarcely later than Balfour's stage b in shark nomencla- 

 ture, is remarkable for the concentration of its elements. Thus, if we compare it 

 in point of size with a similar stage in Pristiurus, measuring it always in terms of its 

 blastoderm, it is of much smaller size. At this stage the length of an embryo of 

 Torpedo measures about one-third the diameter of its blastoderm, that of Pristiurus 

 about one-eighth, and that of Chima^ra not more than one-twelfth. Moreover, 

 its component parts are already more highly differentiated. 



A number of details of this stage are given in plate vi, fig. 39, and figs. 39 a-e. 

 In the first of these (fig. 39) the embryo with its adjacent blastoderm is viewed as 

 an opaque object; it appears next in similar position (a) but as a transparent object, 

 showing ectoderm, entoderm, and archenteron. Behind the embryo the surface of 

 the yolk shows a series of lines representing either surface fissures or vacuoles, 

 related, as we have concluded, to lines of cleavage. In the following figures the 

 embryo is viewed from an antero-dorsal direction (b), postero-dorsal (c), postero- 

 median (d), and postero-ventral (e). The mesoblast is well indicated in plate vi, 

 fig. 39 b, also the extent of the thickening of the ectoblast forming the posterior 

 margin of the embryonic body. In connection with these figures we may refer to 

 the series, fig. 69 a-m, drawn from sections of this embryo cut parallel to the 

 neighboring rim of the blastoderm (/. c, transverse, although slightly oblique to 

 the axis of the embryo), and point out the following features: (i) The size and 

 definiteness of the gut, an important factor in establishing the contour of the 

 embryonic body; the gut acquires the cavity, gc (which communicates with the 

 yolk region only for a short space near the rim of the blastoderm, r, and accumu- 

 lates around its anterior end the bulk of the mesoblast, mcs). (2) The fusion of 

 ecto- and entoblast occurring not merely rt/the tail end of the embryonic body but 



*The circumgerminal zone is, however, shown too distinctly in the present figure, its color should resemble rather 

 that in plate v, fig. 34. 



