The Works of Francis Balfour 2-21

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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.

The Ovum and Spermatozoon | The Maturation and Impregnation of the Ovum | The Segmentation of the Ovum | Dicyemae and Orthonectidae Dicyema | Porifera | Coelenterata | Platyhelminthes | Rotifera | Mollusca | Polyzoa | Brachiopoda | Chilopoda | Discophora | Gephyrea | Chaetognatha | Nemathelminthes | Tracheata | Crustacea | Pcecilopoda | Echinodermata | Enteropneusta | Bibliography
Online Editor 
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This historic 1885 book edited by Foster and Sedgwick is the second of Francis Balfour's collected works published in four editions. Francis (Frank) Maitland Balfour, known as F. M. Balfour, (November 10, 1851 - July 19, 1882) was a British biologist who co-authored embryology textbooks.

The Works of Francis Balfour Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.

Modern Notes:

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Draft Version - Notice removed when completed.

Vol II. A Treatise on Comparative Embryology (1885)

Chapter XXI. Enteropneusta

The larva of Balanoglossus is known as Tornaria. The prselarval development is not known, and the youngest stage (fig. 272) so far described (Gotte, No. 569) has many remarkable points of resemblance to a young Bipinnaria.

A mouth (m\ situated on the ventral surface, leads into an alimentary canal with a terminal anus (an). A prae-oral lobe is well developed, as in Bipinnaria, but there is no post-anal lobe. The bands of cilia have the same general form as in Bipinnaria. There is a prae-oral band, and a longitudinal post-oral band ; and the two bands nearly meet at the apex of the praeoral lobe (fig. 273). A contractile band passes from the oesophagus to the apex of mouth; an. anus, the prae-oral lobe, and a diverticulum (fig. 272, W) from the alimentary tract, directed towards the dorsal surface, is present. Contractile cells are scattered in the space between the body wall and the gut.


W. so-called watervascular vesicle developing as an outgrowth of the mesenteron; m.

In the following stage (fig. 274 A) a conspicuous transverse post-oral band of a single row of long cilia is formed, and the original bands become more sinuous. The alimentary diverticulum of the last stage becomes an independent vesicle opening by a pore on the dorsal surface (fig. 274 A, w). The contractile cord is now inserted on this vesicle. Where this cord joins the apex of the prae-oral lobe between the two anterior bands of cilia a thickening of the epiblast (? a ganglion) has become established, and on it are placed two eye-spots (fig. 273 oc, and fig. 274 A). A deep bay is formed on the ventral surface of the larva.

FIG. 273. YOUNG TORNARIA. (After Miiller.)

m. mouth ; an. anus ; w. watervascular vesicle ; oc. eye-spots ; c.c. contractile cord.

As the larva grows older the original bands of cilia become more sinuous, and a second transverse band with small cilia is formed (in the Mediterranean larva) between the previous transverse band and the anus. The water-vascular vesicle is prolonged into two spurs, one on each side of the stomach. A pulsating vesicle or heart is also formed (fig. 274 B, ht), and arises, according to Spcngel (No. 572), as a thickening of the epidermis. It subsequently becomes enveloped in a pericardium, and is placed in a depression in the water-vascular vesicle. Two pairs of diverticula, one behind the other, grow out (Agassiz, No. 568) from the gastric region of the alimentary canal. The two parts of each pair form flattened compartments, which together give rise to a complete investment of the adjoining parts of the alimentary tract. The two parts of each coalesce, and thus form a double-walled cylinder round the alimentary tract, but their cavities remain separated by a dorsal and ventral septum.

FlG. 274. TWO STAGKS IN THK 1 >KY KI.< >I'M KN I OF TORNARIA. (After Metschnikoff.)

The black lines represent the ciliated hands. m. mouth; an. anus; br. branchial cleft; ///.

heart ; c. Ixxly cavity between splanchnic and

somatic mesoblast layers; 7.-'. watcr-vascvilar vesicle:

v. circular blood-vessel.

Eventually (Spengel) the cavity of the anterior cylinder forms the section of the body cavity in the collar of the adult, and that of the posterior (fig. 274 B, c) the remainder of the body cavity. The septa, separating the two halves of each, remain as dorsal and ventral mesenteries.

The conversion of Tornaria (fig. 274 A) into Balanoglossus (fig. 274 B) is effected in a few hours, and consists mainly in certain changes in configuration, and in the disappearance of the longitudinal ciliated band.

The body of the young Balanoglossus (fig. 274 B) is divided into three regions (i) the proboscidian region, (2) the collar, (3) the trunk proper. The proboscidian region is formed by the elongation of the prae-oral lobe into an oval body with the eyespots at its extremity, and provided with strong longitudinal muscles. The heart (hi) and water-vascular vesicle lie near its base, but the contractile cord connected with the latter is no longer present. The mouth is placed on the ventral side at the base of the prae-oral lobe, and immediately behind it is the collar. The remainder of the body is more or less conical, and is still girt with the larval transverse ciliated band, which lies in the middle of the gastric region in the Mediterranean species, but in the cesophageal region in the American one.

The whole of the body, including the proboscis, becomes richly ciliated.

One of the most important cha- S us WITH FOUR BRANCHIAL racters of the adult Balanoglossus CLEFTS * (After Alex. Agossiz.)

r . m. mouth ; an. anus ; br. bran consists in the presence of respira- chial cleft . hL heart ; IV. watertory structures comparable with the vascular vesicle, vertebrate gill slits. The earliest traces of these structures are distinctly formed while the larva is still in the Tornaria condition, as one pair of pouches from the oesophagus in the Mediterranean species, and four pairs in the American one (fig. 275, br).


In the Mediterranean Tornaria the two pouches meet the skin dorsally, and in the young Balanoglossus (fig. 274 B, br) acquire an external opening on the dorsal side. In the American species the first four pouches are without external openings till additional pouches have been formed. Fresh gill pouches continue to be formed both in the American and probably the Mediterranean species, but the conversion of the simple pouches into the complicated gill structure of the adult has only been studied by Agassiz (No. 568) in the American species. It would seem in the first place that the structure of the adult gill slits is much less complicated in the American than in the Mediterranean species. The simple pouches of the young become fairly numerous. They are at first circular ; they then become elliptical, and the dorsal wall of each slit becomes folded ; subsequently fresh folds are formed which greatly increase the complexity of the gills. The external openings are not acquired till comparatively late.

Our knowledge of the development of the internal organs, mainly derived from Agassiz, is still imperfect. The vascular system appears early in the form of a dorsal and a ventral vessel, both pointed, and apparently ending blindly at their two extremities. The two spurs of the water-vascular vesicle, which in the Tornaria stage rested upon the stomach, now grow round the oesophagus, and form an anterior vascular ring, which Agassiz describes as becoming connected with the heart, though it still communicates with the exterior by the dorsal pore and seems to become connected with the remainder of the vascular system. According to Spengel (No. 572) the dorsal vessel becomes connected with the heart, which remains through life in the proboscis : the cavity of the water-vascular vesicle forms the cavity of the proboscis in the adult, and its pore remains as a dorsal (not, as usually stated, ventral) pore leading to the exterior.

The eye-spots disappear.

Tornaria is a very interesting larval form, since it is intermediate in structure between the larva of an Echinoderm and trochosphere type common to the Mollusca, Chxtopoda, etc. The shape of the body especially the form of the ventral depression, the character of the longitudinal ciliated band, the structure and derivation of the water-vascular vesicle, and the formation of the walls of the body cavity as gastric diverticula, are all characters which point to a connection with Echinodcrm larvae.

On the other hand the eye-spots at the end of the prae-oral lobe 1 , the contractile band passing from the oesophagus to the eye-spots (fig. 273), the two posterior bands of cilia, and the terminal anus are all trochosphere characters.

The persistence of the prae-oral lobe as the proboscis is interesting, as tending to shew that Balanoglossus is the surviving representative of a primitive group.


(567) A. Agassiz. "Tornaria." Ann. Lyceum Nat. Hist.\u\. New York, 1866.

(568) A. Agassiz. "The History of Balanoglossus and Tornaria." Mem. Amer. Acad. of Arts and Stien., Vol. IX. 1873.

(569) A. Gotte. " Entwicklangsgeschichte d. Comatula Mediterranea." Archiv fur mikr. Anat., Bd. xii., 1876, p. 641.

(570) E. Metschnikoff. " Untersuchungen iib d. Metamorphose, etc. (Tornaria)." Zeit.fiir wiss. ZooL, Bd. xx. 1870.

(571) J. M tiller. " Ueb. d. Larven u. Metamor. d. Echinodermen." Berlin Akad., 1849 and 1850.

(572) J. W. Spengel. "Ban u. Entwicklung von Balanoglossus. Tagebl. d. Naturf. Vers. Miinchen, 1877.

1 It would be interesting to have further information about the fate of the thickening of epiblast in the vicinity of the eye-spots. The thickening should by rights be the supra-oesophageal ganglion, and it does not seem absolutely impossible that it may give rise to the dorso-median cord in the region of the collar, which constitutes, according to Spengel, the main ganglion of the adult.

Cite this page: Hill, M.A. (2021, June 15) Embryology The Works of Francis Balfour 2-21. Retrieved from

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