Carnegie Institution of Washington Publication (No. 48)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1906 Excerpt: ... in whose hindmost region only appears the former flattened condition; (2) the appearance of auditory sacs (au); (3) the modeling of optic vesicles (op); the protrusion of the forebrain region into a frontal knob (k). 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 flattened caudal eminence. About 25 somites are present. A third embryo, plate vn, figs. 43 and 43s, shows over sixty somites, and gives us a picture of the young Chimaera at about the end of the first month of incubation. In this stage over sixty somites are present, and the tail bud has grown out conspicuously. The broad flattened trunk terminal of the preceding embryo is here represented, and at a the anal region, a point anterior to which the number of somites corresponds in a general way to that in the earlier stage. Noteworthy advances include: (1) A more definite modeling of the regions of head and trunk. The latter has now lifted up above the surrounding blastoderm, and the head (including the chin region) has separated from the yolk-wall. (2) The gill-slits are now conspicuous, although, as sections show, they have not yet broken through; we note that the spiracular slit s, evidently the equivalent of g in the former stage, is of considerable size; behind it occur three prominent depressions and the trace of fourth and fifth. (3) The appearance of pronephros and pronephric duct; the pronephros itself is situated at the plane of the ninth, tenth, eleventh and twelfth somites, as can better be seen in the transparent prepara...