CHIM^EROID FISHES AND THEIR DEVELOPMENT. 



Fig. 75. Detail of extra-embryonic region of embryo of fig. 73. 

 . Ectoderm ; m, mwoblasl ; meg, meg', gigandc yolk-cells ; v, vacuok; 

 l/e, yollt-entoderm. 



have already seen, to the walls of the gut, since it passes to them yolk masses, 

 large and small, and perhaps also dissolved yolk material. In evidence of the 

 nutritive value of this material witness numerous mitoses in the adjacent (inmost) 

 cells of the entoderm one of which appears in the present section. 



In fig. 75 a detail is given of the 

 process by which yolk-cells are passed 

 into the tissues of the embryo. In this 

 portion of the extra-embryonic blasto- 

 derm the mesoderm occurs only as 

 detached (mesenchymatous) cells (m); 

 the ectoderm forms a single-celled 

 layer, and the entoderm a closely 

 formed cellular mass (ye). Between 

 the entoderm and the yolk is the usual 

 zone of vacuoles (v). At meg a large 

 yolk-filled cell (cf. pp. 83 et set?.) pro- 

 trudes from the yolk into the entoderm, 

 the cells of the latter affording little bar 

 to its progress upward. In this connection we note that the huge cell (meg) lies 

 now within a vacuole in whose wall yolk-nuclei appear; indeed at one point a yolk- 

 nucleus has actually entered the vacuole. In the same figure at meg' is a large 

 cell (cut not quite through the middle) which has evidently had a similar origin to 

 meg; for from its size it can not be confused with a neighboring cell of any germ 

 layer. It contains coarse yolk, and on account of its irregular outline, judging from 

 earlier instances, it has probably undergone division by amitosis. 



ADDITIONAL EMBRYOS OF THIS PERIOD. 



A second embryo of this period, i. e., prior to the breaking through of gills 

 and mouth, is shown on plate vn, figs. 42, 42" and 42**, and on plate vin, fig. 42. 

 The present specimen is badly bent in its trunk region, but in other regards it may 

 be readily compared with the earlier stage, plate vi, fig. 41. The chief advances 

 include: (i) the modeling of the trunk, in whose hindmost region only appears the 

 former flattened condition; (2) the appearance of auditory sacs (au); (3) the model- 

 ing of optic vesicles (op); the protrusion of the forebrain region into a frontal knob 

 (). The general shape of the head, as shown in dorsal view, already suggests the 

 adult condition, in spite of the small size of the embryo. This now measures only 

 2. 5 mm. , not allowing for the bent trunk region. The tail at this stage protrudes 

 beyond the rim of the blastoderm, its tip budding out like a knob beyond the flat- 

 tened caudal eminence. About 25 somites are present. 



A third embryo, plate vn, figs. 43 and 43", shows over sixty somites, and 

 gives us a picture of the young Chimsera at about the end of the first month of 

 incubation. In this stage over sixty somites are present, and the tail bud has 



