132 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



layered during the early stages of otocyst development the super- 

 ticial layer is seen to pass over tin- rudiment unaltered (Fig. 73, A). 

 In Lung-fishes the cavity of tin- ntoeyst appears secondarily in the 

 midst of an apparently solid downgrowth >i' the deep layer of the 

 ectoderm (Fig. 73, B) but the examination of earlier stages (Fig. 

 73, A) shows that here also then- is an actual involution of the deep 

 layer although there is at first no patent cavity. 



In the Elasmobranchs the otocyst retains throughout lite its con- 

 iii-xion with the exterior, the connexion becoming drawn out into a 

 slender tube. In Birds the recess, and therefore the endolyniphatic 

 duct, represents the remains of this original connexion, but curiously 

 enough in certain other Vertebrates e.g. Lung-fishes (Fig. 73, C) the 

 last connexion of otocyst with external ectoderm lies lateral of and 

 somewhat anterior to the eudolymphatic duct which latter here 

 develops as an independent outgrowth of the otocyst wall. This is 

 to be looked on as a secondary modification of the more primitive 

 arrangement seen in Elasmobranchs. The structure- named endo- 

 lymphatic duct in Teleosts also arises as a secondary outgrowth of the 

 otocyst wall. 



The endolyniphatic duct or recess commonly persists in the adult 

 as a conspicuous blindly ending diverticulum of the otocyst wall. In 

 Lung-fishes and Amphibians its wall proliferates actively giving rise 

 to projections which in the Lung-fishes and some Urodeles, e.g. the 

 Axolotl, meet to form an irregular sac over the roof of the fourth 

 ventricle. In the Anura the irregular thin-walled sac formed in this 

 way spreads forwards and also laterally until it becomes continuous 

 ventrally so as completely to surround the hind-brain. An unpaired 

 prolongation of this sac extends tailwards immediately dorsal to the 

 spinal cord within the vertebral canal. Paired outgrowths of this 

 extend outwards along witli each spinal nerve and expand at their 

 ends round the spinal ganglia to form the calcareous bodies so con- 

 spicuous in the adult frog. The whole system of outgrowths is con- 

 spicuous in the adult from the white otolithic particles in its interior. 

 Tin- vertebral portions become eventually broken up into a network 

 of irregular tubes which is interpenetrated by a network of capillaries 

 & 1889). 



Somewhat similar outgrowths of the endolyniphatic duct make 

 their appearance in Sauropsida although in this case they do not 

 undergo the wide extension that they do in the Anura. In the 

 however they do heroine extended so as to form a large 

 superficially plaeed irregularly lobed sac which covers over a great 

 part of tip- neck region close under the skin (Wiedersheim). 



LATKI;AI. LINK ORQAN8. These sense-buds (neuromasts), which 

 are found arranged in rows on the head and body of lishes and 

 aquatic, amphibians, take their origin in linear thickenings of the 

 dec], layer of the ectoderm which spread along the surface .if the 

 head and bndvand eventually b'eome segmented up into separate 

 pieces. In correlation with the function of these organs, which 



