450 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
through the caudal mass (fig. 74) that the mesoderm can not be distinguished over 
the whole yolk. 
The ectoderm forms a layer two to three cells deep over the whole egg. The outer 
layer of cells is in places well separated from the inner layers, the nuclei are slightly 
larger than those of the inner layers, and the cytoplasm stains lighter. The neural 
thickening consists of a solid ridge of cells extending down from the ectoderm to the 
notochord. From the sections it would appear that there is little difference between 
different points along this region. The ridge appears deeper just in front of the noto- 
chord than elsewhere, and here the lower layer of cells has a columnar arrangement. 
The diameters of these embryos are respectively 0-27 mm. and 0-3 mm. The 
embryo has grown till it fills the shell and with but a slight further increase it hatches, 
the membrane bursting with the further expansion of the embryo. In other teleosts 
the hatching process is largely due to the muscular efforts of the embryo, while in 
this case the muscle cells can scarcely, if at all, be distinguished from other cells at the 
time of hatching. The yolk measures 158 p to 200 jj. 
I have not been able to secure stages between this and the embryo with six ( ?) or 
more protovertebrse, which, is hatched. 
General development of the larvae . — At the time of hatching, the tail has not begun 
to bud out. The larva encircles the entire yolk and is so transparent that it can only 
be found with a lens. Groups of spermatozoa are found attached to its surface. No 
lumen has appeared in the intestine and muscles have not begun to be differentiated, 
so that the larva is entirely incapable of motion. 
Shortly after hatching, the head and tail, exteriorly almost identical, sometimes 
overlap, the body being bent over the yolk (fig. 83). However, the larva rapidly 
straightens itself if this overlapping is normal at all. The lumen of the intestine 
appears when 10 protovertebrse have been formed (fig. 77) and Kupffer’s vesicle has 
become enormously enlarged. The muscle cells have now become somewhat elongate, 
though motion is probably not yet possible. The tail is not yet free and the larvse 
measure about 0-6 mm. By the time the larva has reached a length of 0-8 mm. the tail 
forms a thick blunt projection beyond the yolk and the intestine has a continuous lumen 
from near the anterior end of the notochord back to the future anus (figs. 78 and 84). 
The heart is represented by a large mass of cells below the head and about 12 proto- 
vertebrse have been formed. A swelling is evident in the gill region and shortly after- 
wards (by the time the larva is 0-9 mm. long) the first gill-slit is opened. The eye and 
auditory organs now become more prominent, but no otoliths are formed as yet. About 
14 protovertebrse are present in larvse 0-9 mm. long. Spermatozoa are now found in the 
intestine, which they reach through the first gill- slit. 
If larvse are examined from this stage till the mouth is formed a stream of the 
ovarian fluid is usually seen entering the gill-slit and escaping through the anus. 
This stream is kept in motion by very active cilia in the gullet. 
When the larva has reached 1 mm. in length, the tail is formed to a considerable 
extent and terminates in a blunt lobe. The hypertrophy of the hind gut has begun and 
reaches some distance beyond the yolk-sack. One of the most peculiar structures of 
the larva, the enormous yolk-sack, has now become well developed. The yolk is 
quite minute and lies at the posterior end of this cavity, and all the remaining yolk- 
sack is utilized as a pericardium (figs. 88 and 90.) The heart is now a simple, slender 
