366 
MESSRS. P. M. BALFOUR AND W. N. PARKER ON TILE 
apparent. They form shallow pits (Plate 22, fig. 24, cm,) of the thickened part of the 
nervous layer adjoining the brain in this region. Each pit is covered over by the 
epidermic layer above, which has no share in its formation. 
In many parts of the lateral regions of the body the nervous layer of the epidermis 
is more than one cell deep. 
The mesoblastic plates are now divided in the anterior part of the trunk into a 
somatic and a splanchnic layer (Plate 22, fig. 25, so., sp.), though no distinct cavity 
is as yet present between these two layers. Their vertebral extremities are some¬ 
what wedge-shaped in section, the base of the wedge being placed at the sides of 
the medullary keel. The wedge-shaped portions are formed of a superficial layer of 
palisade-like cells and an inner kernel of polygonal cells. The superficial layer on the 
dorsal side is continuous with the somatic mesoblast, while the remainder pertains to 
the splanchnic layer. 
The diameter of the notochord has diminished, and the cells have assumed a 
flattened form, the protoplasm being confined to an axial region. In consequence of 
this, the peripheral layer appears clear in. transverse sections. A delicate cuticular 
sheath is formed around it. This sheath is probably the commencement of the per¬ 
manent sheath of later stages, hut at this stage it cannot be distinguished in structure 
from a delicate cuticle which surrounds the greater part of the medullary cord. 
The hypoblast has undergone no changes of importance. 
The layers at the posterior end of the embryo retain the characters of the last stage. 
Sixth clay after impregnation .—At this stage (Plate 21, fig. 8) the embryo is con¬ 
siderably more advanced than at the last stage. The trunk has decidedly increased in 
length, and the head forms a relatively smaller portion of the whole. The regions of the 
brain are more distinct. The optic vesicles (op.) have grown outwards so as to nearly 
reach the edges of the area which forms the parietal part of the body. The fore-brain 
projects slightly in front, and the mid-brain is seen as a distinct rounded prominence. 
Behind the latter is placed the hind-brain, which passes insensibly into the spinal 
cord. On either side of the mid- and hind-brain a small region is slightly marked off 
from the rest of the parietal part, and on this are seen two more or less transversely 
directed streaks, which, by comparison with the Sturgeon,* we are inclined to regard 
as the two first visceral clefts (br.c.). We have, however, failed to make them out in 
sections, and owing to the insufficiency of our material, we have not even studied 
them in surface views as completely as we could have wished. 
The body is now laterally compressed, and more decidedly raised from the yolk than 
in the previous stages. In the lateral regions of the trunk the two segmental or 
archinephric ducts (sg.) are visible in surface views : the front end of each is placed at 
the level of the hinder border of the head, and is marked by a flexure inwards 
towards the middle line. The remainder of each duct is straight, and extends 
* Salensky, “Reclierch.es s. le Developpement du Sterlet.” Archives de Biol., vol. ii., 1881, ph xvii., fig. 27. 
