324 
BULLETIN OF THE BUREAU OF FISHERIES. 
individual variation, but commonly elongates transversely to the long axis of the future 
embryo, endures 4 or 5 days, and then completely disappears. In front of the pit 
a wide embryonic area is defined by rapid divisions of the surface cells. The latter, 
which are the direct descendants of the enormous yolk pyramids or hillocks, become 
distinctly separated into a single stratum of yolk-laden and columnar cells. Below the 
point of invagination the ingrowing plug of cells expands by rapid divisions of its 
elements, and like columns of smoke from a steam engine a dense cloud-like mass is 
spread into the yolk. Many of the cells break loose from the syncytial mass and worm 
their way through the yolk like independently moving amoebae. Many of them degen- 
erate, while others creep forward and attach themselves to the embryonic area. The 
cells introduced by invagination give rise, in terms of the germ-layer theory, to the 
hypoblast or endoderm, and to at least a part of the mesoblast. It is almost certain 
that the yolk-wandering cells receive many recruits from the surface of the embryonic 
area; the yolk cells introduced earlier for the most part degenerate before the stage 
of invagination is reached. By multiple divisions eell nests are formed, particularly 
in the embryonic region at the surface, or more commonly just beneath it in the midst 
of spheroidal masses or balls of yolk. 
Death waits close upon the birth of new cells, and from an early stage to the later 
egg-nauplius period degeneration is a marked characteristic of this and many other 
arthropod embryos. Nebulous clouds of chromatin strew the paths of cell migration, 
and are carried to every part of the egg, where they remain until absorbed. In the 
early stages at least embryonic layers do not exist, and attempts to reconstruct them 
out of a mass of rapidly multiplying, degenerating, and moving elements, by the aid 
of theory and the imagination, have thus far proved neither successful nor profitable. 
The appendages are the first of the distinctly embryonic parts to make their 
appearance; they are formed by paired tubular folds of the body wall. They pos- 
sess solid yolk cores which are gradually absorbed and replaced by mesoblastic cells 
which migrate from the embryonic region. The limbs arise in pairs in the following 
order: (i) First antennae, (2) mandibles, (3) second antennae, (4) first maxillae, and 
the remaining thoracic appendages in regular succession. The second antenna soon 
becomes bilobed, the inner branch representing the future long “whip” or flagellum 
of this limb. The first antennae remain single until shortly before hatching, when 
the inner flagellum buds out from the inner lower surface of the primary stalk (see 
p. 226). The optic disks, at first paired rounded areas of rapidly dividing cells, soon 
become elevated into lobes and form the rudiments of the large eyestalks. The 
mouth appears at about the ninth day as a median pit on a line drawn through the 
hinder margins of the buds of the first antennae and before the second antennae are 
formed. At the tenth day the three pairs of nauplius-appendages are present as 
buds; a day or two later the upper lip or labrum has grown down over the mouth and 
a larger fold representing the abdomen and a part of the thorax has grown forward 
from the region of the thoracic-abdominal plate, marked by the earlier point of invagina- 
tion. At 14 days of age the latter fold is divided at its extremity, which represents 
