THE STURGEONS AND STURGEON INDUSTRIES. 
259 
sides curved inward, so that the whole reminds one of the form of the body of a 
violin, with the embryo lying in the middle line, or lengthwise, in the paler central 
area. 
While these changes have been in progress, the cap or mantle of smaller cells 
from which the embryo is formed, and which now almost completely envelops the yolk, 
has undergone internal changes. These relate mainly to the splitting or folding of its 
constituent cells in such a manner, over the embryonic portion especially, as to form 
two layers. From the outer of these the skin and nervous system of the future stur- 
geon will be formed, while from the inferior and thicker layer the rudiments of the 
muscles, blood-vessels, intestine, cartilage, etc., of the future completely developed 
animal will be evolved. 
The changes which follow, and which are visible from the exterior, relate partly 
to the outermost and partly to the innermost layer of cells which cover the yolk. On 
either side of the embryo, in Fig. 7, there are distinct longitudinal swellings, W G. 
These indicate the beginnings of the kidneys, which are still more distinctly visible 
in Fig. 8 as a pair of narrow bauds. In this figure further changes are taking place 
in the fore part of the medullary plate; the anterior and posterior parts of the brain 
are in fact becoming defined, while still further back the four pairs of blocks of cellular 
substance mark the rudiments of as many segments or flakes of the muscular system 
of the adult. Behind these the thickened rim of tissue surrounding the blastopore 
will furnish additional muscular segments as development proceeds, while it is also 
obvious that the rudiments of the-Tosterior limbs of the kidneys embrace this ring of 
tissue. 
These points are still better shown in Fig. 9, in which the medullary tube or rudi- 
mentary spinal canal has become folded off from the muscular segments at the sides, 
and which have greatly increased in number. The whole embryo has also lengthened, 
so that only the posterior portions of lateral rudiments of the head, h p are visible, 
while the blastopore has closed posteriorly. Additional muscular segments are also 
seen to be progressively split off, from before backwards, from the rim of tissue which 
in the preceding stage surrounded the blastopore. The Wolffian ducts or embryonic 
renal apparatus is also seen to extend to or beyond this region. 
Still more advanced stages are represented in Figs. 3, 4, 10, and 11. In Figs. 3 
and 10 the first visceral arches va'^ va", va'" are obvious, while the first outward signs 
of the sense organs, the ear, eye, and nasal pits are plainly visible in Fig. 3. 
In Figs. 4 and 11 are shown very nearly parallel stages of development in the 
common sturgeon and sterlet. Fig. 4 is from below, and Fig. ll shows the head end 
of the embryo of the sterlet from above. In the latter the anterior ends of rudiments 
of the kidneys or Wolffian ducts are seen to have their anterior ends more recurved 
than in the less advanced stages shown in Figs. 9 and 10. At this time the head 
becomes obvious at the anterior end of the embryo, extending some distance forward 
beyond the fore part of the head, as a pulsating tube, which receives the embryonic 
blood from a paired system of vessels extending over the yolk. 
Fig. 12 shows the just-hatched Eussian sterlet, while Fig. 18 shows the common 
sturgeon just after it has left the egg, on the sixth day after impregnation. The yolk 
in the sterlet is now much larger in proportion to the body than in the common stur- 
geon. There does not seem to be any sign of the pectoral fin developed in the just- 
hatched sterlet; but in the common sturgeon this fin is the first of the paired ones to 
