216 



Part III. — Suieenth Annual Report 



tlie embryonic trunk (mesoblast and nervous system). In the embryo of 

 anchovy this space is especially wide. Corpuscles were seen in this space 

 in the larva of Scorpcena species. The space is evidently filled with a 

 fluid of some nature, and, in certain embryos — e.g.^ anchovy, Mugil, etc. — it 

 seemed to be connected with the gut by the intermuscular spaces. A not 

 improbable hypothesis is that the perivitelline fluid which enters the gut 

 also circulates in this space ; but at present no direct proof is adducible in 

 support of this suggestion. 



The respiration of the embryo has not hitherto been regarded as in- 

 volving any special system of vessels. Respiration is supposed to take 

 place directly through the tissues o.t every part of the embryo — a process 

 of osmosis. Whether such a method is sufficient for complete aeration in 

 certain eggs is not of material moment ; but that that method is not the 

 sole one is shown by the existence of the specialised respiratory system of 

 which the gut is the principal organ. In the embryo the whole gut is 

 used for respiration ; in the young fish the pharynx is supplied with 

 special respiratory organs — viz., gills — and through them alone respiration 

 is carried on. In the embryo the perivitelline fluid probably circulates in 

 the sub-epidermal space, to which it may obtain entrance by the spaces 

 between the muscle segments or by the anterior prolongation of the gut 

 seen in certain embryos. A fuller examination of the respiratory system 

 is required before its exact circulation can be determined. 



Lateral Sense Organs. 



The name "lateral sense organs" has been given to the small papillae 

 which appear, just before hatching, on the sides of the embryo. They are 

 usually paired — the papilla on one side being exactly opposite a corres- 

 ponding papilla on the other side. On the tail, however, the papilljB are 

 are not all paired ; the most p;)sterior papilla is usually single. 



The lateral sense organs in the larva are described as papillae bearing 

 externally a number of palpocils or sensory hairs. Ryder drew attention 

 to them in the cod (Gadus morrlma), anil mentioned that each papilla 

 receives a small nerve tw^ig from the spinal cord ; this nerve crosses the 

 sub-epiblastic space. 



In the embryo of anchovy the lateral papillae are well seen {Lp., fig. 8). 

 Small pear-shaped corpuscles project from the papillae. A long proto- 

 plasmic process was attached to the apex of one papilla (Ip^., ih.). The 

 summit of the papilla usually has a small depression or dimple in it. The 

 papillae gradually diminish in size as they near the end of the tail. 



In a larva of anchovy, just hatched, the small pear-shaped corpuscles 

 were attached to the papillae. Behind the anus were two pairs of these 

 organs, and of these the more posterior pair, though the smaller, had a 

 double form. In the anchovy larva, however, there is usually a delicate 

 tube projecting from each pa])illa at right angles to the side of the embryo 

 '{l.p.y fig. 26). The distal poition of the tube is bent backwards. There 

 are seven papillae on the right and six on the left side. The first six 

 papillae on each together form pairs ; the seventh is unpaired. In the 

 larva of Species No. 3 (Raff.) there are five papillae on each side {l.p., fig. 

 10) ; the first two form pairs ^vith their fellows on the opposite side ; the 

 remainder do not form pairs. The anterior three papillae on each side are 

 alone furnished with external tubes. Small lateral papillae are present in 

 the larva of Mugil species. One papilla had a pore-like depression in its 

 summit, while a second had a corpuscle attached to its apex. 



Almost all the tubes have a number of small granules in their distal 

 portions (Z.^., fig. 26) ; these granules remain quite stationary. The fibres 



