Memoirs: The Development of Symbranchus marmoratus

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Abstract
(A) General Features of Development. (1) The egg of Symbranchus is small, its development typically Teleostean and rapid, the larva hatching out in about seven days at a tropical temperature. (2) A rostrum appears just before the larva hatches, increases in size, attains a maximum length of about 1 mm. when the creature is 7 mm. long, decreases in size, gradually dying down to a rounded pad, and eventually disappears just before tbe adult stage is reached. (3) The larva possesses pectoral fins and the shoulder girdle persists in the adult. These fins appear early, are muscularised by the first three trunk myotomes and innervated by the first three spinal nerves. They develop rapidly, reach their maximum size seven or eight days after hatching, shrivel somewhat, and then drop off bodily at Stage 34. The pectoral fins are mainly respiratory organs and possess a rich network of capillaries. There are three principal blood-streams in the fins--one central, afferent, two marginal, efferent. The establishment of perfect branchial respiration is coincident with the falling off of the fins, i. e. when the creature is ten days old. (4) No trace of pelvic fins has been found. (5) Perforated gill-slits of the Elasmobranch type do not occur in early stages, the clefts only becoming perforate after they are covered by the operculum. When branchial respiration is just beginning the gill-chamber opening is a single crescent-shaped one; as development proceeds the anus of the crescent are gradually obliterated, owing to the fnsion of the backwardly growing operculum with the body-wall, and a single median ventral opening is the result. (6) There is a blind diverticulum in the dorsal roof of the mouth behind the hyoid. (B) Alimentary Canal. (1) The alimentary canal has a typical Teleostean character and development, is solid at first, hollowed out secondarily, and has no obvious connection with the yolk. (2) No air-bladder has been detected at any stage. (3) The pyloric valve arises by outpushings of the intestine. These blind cæcal outgrowths have the appearance of very short rudimentary pyloric cæca. (4) Apart from these structures there are no pyloric cæca. (5) The pancreas is an elongated compact gland arising from a dorsal and two ventral rudiments. (6) The liver is elongated and unilobed. (7) There is a typical thymus arising from clefts 2, 3, 4 and 5. (8) A thyroid arises as a solid median derivative of the floor of the pharynx. It is elongated and bilobed anteriorly. (9) The spleen develops early, is very conspicuous, and multilobed at first. (c) Renal Organs. (1) The pronephric chamber and tubule are formed from the nephrotome of the third trunk myotome. (2) There is no communication at any time between splanchnocœle and nephrocœle of the pronephros. (3) The archinephric duct is formed from the nephrotomes of the segments posterior to the third; the conversion of these nephrotomes into a duct takes place simultaneously, involving no backward growth of the archinephric duct. (4) The pronephros is still present in the oldest larva examined. (5) Mesonephric tubule-rudiments appear in Stage 29. They occur from about Segment 25 to Segment 43. Bach arises as a rounded clump of darkly stained cells in the immediate neighbourhood of the archinephric duct. This rudiment is gradually moulded into a twisted tubule, one end of which becomes converted into a Malpighian capsule of the usual type, the other end acquiring an opening into the archinephric duct. (6) There are no peritoneal funnels. (7) Secondary mesonephric tubules arise in connection with the archinephric duct and with the primary mesonephric tubules. These are not fully differentiated in the oldest larva examined. (8) The anterior much-coiled part of the archinephric duct, as well as the mesonephros, is surrounded by pseudolymphatic tissue. (D) Vascular System. (1) The development of the heart and vascular system agrees generally with that described for other Teleosteans. (2) The free anterior part of the left posterior cardinal disappears, the large right posterior cardinal conveying the blood of the inter-renal vein to the heart. (3) There is a close connection between the blood-vessels of the hinder ends of the kidney and liver recalling the posterior vena cava of Polypterus. (4) The subintestinal vein, the front end of which is the vitelline vein of the earlier stages, persists in the adult as a hepatic vein. This hepatic vein joins up with the left anterior cardinal and left jugular to form the left ductus Cuvieri. The right ductus Ouvieri shows no special peculiarity. (E) Nervous System. (1) The brain is at first solid and is hollowed oat secondarily. (2) Three main divisions of the brain can be distinguished in Stage 21. (3) There is tio cranial flexure until Stage 24, and therefore no reason for assuming that the iufundibulum is the morphologically anterior end of the brain. (4) Sagittal sections through the brain at different stages show the usual Teleostean characters. (5) The cerebellum is late in developing and goes on growing after metamorphosis. (6) The optic lobes of the mature brain are relatively smaller than in the developing one. The mid-brain of the adult is the least conspicuous part. (7) The mature brain is elongate, as also are the olfactory and optio nerves, the divisions well separated off.

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