ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
319 
a typical ovum, have, of course, nothing to do with segmentation. The 
development of the “vitelline membrane” has not been satisfactorily 
worked out. The segmentation- cavity of the segmented Teleostean 
ovum is homologous with that of other ova. Accepting the conclusions 
of Agassiz and Whitman in regard to the origin of the nucleated peri- 
blast, Mr. Cunningham adheres to his own observation that, as the 
nuclei from the marginal cells from the sixteen-cell stage onwards con- 
tinually divide, cell-division also takes place in these cells, but at a 
slower rate than the nuclear division. In consequence of this, new cells 
are continually being separated from the ring of periblast at the same 
time that the nuclei in that ring continually become more numerous, and 
extend outwards and inwards from the marginal region of the blasto- 
derm. The segmentation of cells from the Teleostean periblast to form 
hypoblastic and mesoblastic tissues corresponds perfectly with the sub- 
division of the yolk-cells in Petromyzon and Amphibians which gives 
rise to hypoblast and mesoblast in these forms. The real representative 
of the gastrula-cavity in Teleosteans is Kupffer’s vesicle. 
Later Larval Development of Amphioxus.* — Mr. A. Willey gives, 
at the commencement of his paper, a useful resume of the entire develop- 
ment of Amphioxus. 
I. The period of embryonic development comprises the first thirty- 
two hours. It commences with the segmentation of the ovum, and ends 
with the formation of the first gill-cleft. 
( а ) In the first eight hours, during which the embryo is confined within 
the vitelline membrane, the usual early differentiations are effected. 
(б) After emerging from the membrane there is a successive forma- 
tion of myocoelomic or archenteric pouches to the number of fourteen 
pairs. The myotomes which are added after this period never communi- 
cate with the intestine. 
II. In the period of early larval development fresh gill-slits appear 
metamerically, slightly to the right side of the median line (subsequently 
passing well up to the right side) to the number of twelve to fifteen. 
Towards the close of this period the longitudinal metapleural folds 
appear, and the closure of the atrium commences behind by the fusion of 
the small subatrial ridges which are developed in the inner face of the 
metapleura. 
III. The period of later larval development is that in which the 
second row of gill-slits is formed on the right side ; the first or primary 
row crosses to the left side, the mouth assumes an anterior median and 
vertical position, the preoral cirri appear, and the endostyle is develoj>ed 
from its pre-existing rudiment. 
IV. The adolescent period is marked by the attainment by the young 
Amphioxus of most of the essential features of adult structure ; it now 
definitely ceases to lead a pelagic life and takes up its abode in 
the sand, where its further growth in size and maturity is accom- 
plished. 
The third period, or that now described, is divided into eight 
stages, in the consideration of which it is necessary to distinguish 
between what takes place on the right and what on the left side. 
♦ Quart. Journ. Micr. Sci., xxxii. (1891) pp. 183-231 (3 pis.), 
z 2 
