566 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



siren — remained unknown so far as its development was concerned 

 until 1896 when Graham Kerr succeeded in obtaining abundant 

 embryological material in the Gran Chaco of South America. 



The developmental stages of Protopterus, the next representative 

 of the group to become known to science, were first obtained on the 

 Gambia Eiver by Budgett who had taken part in the Lepidosiren 

 expedition a few years earlier. Ceratodus, the last of the surviving 

 genera to become known in the adult condition, was the first to be 

 made known embryologically by Caldwell and Semon as already 

 mentioned (p. 435). 



The Lung-fishes like other animals living under similar conditions 

 breed at the commencement of the rainy season (Protopterus, Gambia, 

 August ; Lepidosiren, Chaco, November but incidence of rainy season 

 irregular and may be delayed — till e.g. June — or omitted altogether ; 

 Ceratodus, September to December). In the case of Ceratodus the 

 eggs are scattered loosely about amongst the water plants, while in 

 Protopterus and Lepidosiren they are deposited in a special burrow at 

 the bottom of the swamp where they are guarded by the male parent. 



Dipnoans live well in captivity and there is little doubt that it 

 will be found easy to induce them to breed by using similar methods 

 to those described under the heading Amphibia. It is particularly 

 desirable that this should be done in the case of Lepidosiren on 

 account of the large size of its histological elements which make it a , 

 peculiarly suitable type for the investigation of various problems of 

 histogenesis. 



The eggs of Dipnoi, especially of Lepidosiren, are of large size and 

 this makes it especially advisable to use celloidin in addition to 

 paraffin methods of embedding. When paraffin is used it is necessary 

 to remove the egg envelope by slitting it up with fine scissors, care 

 being taken to keep the point of the scissors close to the envelope so 

 as to avoid injury to the surface of the egg. 



Corrosive sublimate and acetic acid is a good stock fixing agent. 

 Eor stages before hatching 10% formalin is convenient. 



Amphibia. — The most easily obtained embryological material is 

 that of the common Erogs of the genus Rana the masses of spawn of 

 which are familiar objects in pools during the early weeks of spring, 

 in temperate climates. The exact time differs with climate and also 

 with species, some species such as R. esculenta in Europe and R. 

 catesbiana in North America lagging several weeks behind the others. 

 The spawn, fertilized as deposited in the early morning, may con- 

 veniently be kept during its development in earthenware pans. The 

 water should be left stagnant and unchanged during the period prior 

 to hatching as under these circumstances the spawn is less liable 

 to be attacked by fungus but the hatched larvae should be at once 

 transferred to clean water. 



Investigations are greatly needed on the embryology of Anura 

 outside the genus Rana (cf. Figs. 251, 252, 253 and 254). The 

 different genera and species differ greatly in the size of the egg 



