BY H. L. KESTEVEV. 53 



egg, and has led to very fallacious conclusions. That this 

 method of determination is entirely unreliable, is illustrated 

 by the following facts. In most fishes, the ova are shed before 

 fertilisation, but, in many of the sharks and rays, the off- 

 spring are born as fully-formed young ; whilst, in others of 

 the Selachians, the fertilised ovum is deposited in the early 

 stages of segmentation enclosed in a horny shell. The great 

 majority of the Amphibia deposit eggs, and the young go 

 through a free tadpole stage ; two Salamanders are vivi- 

 parous, and, in one of these, the young are liberated in the 

 form of the adult, as also are the young of Pipa americana. 

 Most Reptilia are oviparous ; one or two Lacertilians and 

 Ophidians are ovoviviparous. It follows, then, that no 

 generalisation may be made on ontogenetic stages, which 

 takes as its basis the period of hatching or liberation by the 

 maternal organism. 



It is unsafe to found identifications on the presence or 

 absence of specific characters ; this practice has also led to 

 errors. To the Ornithologist, the unsegmented ovum would 

 be specifically recognisable, by its shell ; in fact, in nearly all 

 cases, the ova may be identified by the specialist ; they 

 already bear specific characters. 



Buckman and Bather (4) defined the Ncpionic stage as 

 that immediately succeeding the embryonic stages, and dur- 

 ing which no specific characters make their appearance ; and 

 the next, or Neanic stage, as that during which specific 

 characters and all other morphological features present in 

 the adult, appear and undergo development. Even if, as is 

 probable, these authors meant specific characters of the adult 

 organism, the definitions are not capable of general applica- 

 tion ; there are instances among the Gastropoda where they 

 fail to lead us right ; and throughout the whole of the 

 Amniota, practically all the specific characters of the adult 

 are developed before the loss of the larval organs. 



In view of the fact that the degree of development of the 

 different organs and systems of organs is subject to wide 



