CH.vu TIIK EXTERNAL FKA'ITIMvS OF THE BODY 433 



(lanoids Amiii ;iii(l Li-/n'i/ux/<'tix the main mass of volk retain 

 form for a considerable period, causing a gjeal liiil-in- of tin- \eiitral 

 body-wall anteriorly. In the Teleost this is .still further accent- 

 uated, the bulging forming (he yolk-sac which remains -prominent 

 even in larvae sufficiently developed to he al>le to swim acti\el\ . 

 An extreme case of the prominence of the yolk-sac is afforded by 

 (,'i/ininii'i-Jmx (Fig. liMJ) where it shows a peculiarly elongate* 1 I'orm 

 tor a certain j.eriod. Cement organs are, as a rule, absent in Teleostei : 

 so also are external gills though in rare cases the latter have physio- 

 logical representatives in filamentous prolongations of the -ill 

 lamellae ( Fig. 1!)!). 1'. . 



Great \ariety of form exists amongst the larvae of Teleostean 

 fishes, more especially amongst those of pelagic hahit. Familiar 

 examples are seen in the pelagic larvae of. the lv-ls much com- 

 pressed from side to side, transparent and colourless even the blood 

 being tree from haemoglobin and much greater in hulk than the 

 immediately succeeding phase in the, life-history. The larvae of the 

 Flat-fishes (Pleuronectidae Flounder, Plaice, Sole, etc.) are again of 

 special interest owing to the extraordinary asymmetry which they 

 develop. They are at first quite symmetrical and in no way abnormal. 

 The larva swims at this time with its laterally compressed body 

 vertical alter the manner of a Bream but later develops the habit 

 of swimming on its side. The side of the head-region which is below 

 now grows more actively than the other so that the head becomes 

 strongly asymmetrical and the eye of the lower side becomes gradu- 

 ally transferred to the upper, the right and left eyes being now both 

 on the same side of the head. Correlated with this asymmetry in 

 form there comes about a corresponding asymmetry in colour, the 

 chromatophores being collected together on the upper side and 

 giving it its characteristic obliterative colouring. In some genera it 

 is the right side of the body which is above, in others the left while 

 in a few species it appe irs to be indifferently the one or the other. 



DIPNOI. Both of the dipneumonic Lung-fishes Lepidosiren and 

 Protopterus have been investigated (Graham Kerr, 1900 and 1909 : 

 I Judge tt, 1901). They closely resemble one another and Lepidosiren 

 will be chosen here for description (Fig. 200). 



During the early stages of the modelling of the embryonic body 

 (Fig. 200, A) the latter is curved round the egg, occupying about 

 290 in angular extent. The head-region becomes demarcated as a 

 slight, somewhat lance-shaped protuberance above the general surface 

 of the egg due to the neural rudiment. The branchial region becomes 

 marked at an early stage by a slight elevation of the sin-face which 

 Siion becomes divided by shallow oblique grooves into the sen- 

 branchial arch rudiments. About stage 25 ( Fig. L'uO. B) the tip of the 

 head and the tip of the tail project sharply above the general surface: the 

 external gills (r.//) are now in the form of tour distinct little kno 

 each side, and the cement organ (c.n) has made its appearance as a 

 civscentic structure on the ventral side curving round tin- tip of the 



VOL. II 1' 1- 



