20 HEMICHORDATA CHAP, 
alimentary canal. This space becomes obliterated for the most 
part by the enlargement of the body-cavities, and its last remains 
persist, as in many other animals,’ as the vascular spaces of the 
adult. 
In Dolichoglossus howalevskii, and probably in other species 
with large eggs,’ development proceeds by gradual stages to the 
adult form, and no Tornaria-stage is passed through. The opaque 
young animal, on being hatched, creeps about in the muddy sand 
in which the adult is found, later moving in a leech-like manner, 
by alternately attaching itself by its two ends. The young 
stages were ingeniously obtained by Bateson, to whom our 
knowledge of the development of this species is due? by allowing 
a large quantity of the mud to settle after being stirred up, the 
layer of the specific gravity corresponding with that of the young 
Balanoglossus being then separated by means of a siphon. The 
young stages previously contained in several hundredweight of 
mud were thus easily collected into a pint of water. Morgan 
recommends treating the layer obtained by a similar process with 
picric acid, which stains the young Balanoglossus yellow. 
The embryo early becomes a “ blastosphere ” or hollow vesicle 
formed of a single layer of cells. One half of this is invaginated, 
or pushed into the other half, and a “ gastrula ” is thus developed, 
the cavity of which is the “archenteron,” and the two cell- 
layers respectively “ectoderm” and “endoderm.” The “ blasto- 
pore,” or orifice of invagination, is at the posterior pole of the 
larva, where it narrows and closes, the locomotor, transverse band 
of cilia developing round it. No other bands of cilia appear in 
this form of development. The proboscis becomes marked out 
externally by the appearance of a circular groove, near the 
middle; and behind this groove a second one appears, which 
forms the posterior boundary of the collar. The larva, which 
now resembles Fig. 8 C, is usually hatched at this stage. Two 
gill-slits make their appearance, and the mouth and anus are 
perforated ; the anus being in the position of the blastopore. 
1 For Vertebrates see Shipley, Quart. J. Micr. Sci. xxvii. 1887, p. 340. 
° The largest known eggs are those of Hurrimania kupfferi (1°3 mm.). The 
eggs of Dolichoglossus kowalevskii measure *37 mm., while the youngest Tornaria 
found by Morgan, already transparent and with their tissues distended by water, 
were only about two-thirds that size. 
3 Quart. J. Mier, Sci. xxiv. 1884, p. 208; xxv. Suppl. 1885, p. 81; xxvi. 1886, 
pp. 511, 535. 
