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, 1 as the vascular spaces of the 

 adult. 



In Dolichoglossus kowalevskii, and probably in other species 

 with large eggs, 2 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, 3 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. 



2 The largest known eggs are those of Harrimania 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. Micr, Sci. xxiv. 1884, p. 208 ; xxv. Suppl. 1885, p. 81 ; xxvi. 1886, 

 pp. 511, 535. 



