54 THE CONSTITUTION OF THE GASTROPOD PROTOCONCH, 



variation, it seems that some character must be chosen arbi- 

 trarily and used as a datum-line from which to measure the 

 stages on either side. Thus we find that, in one group, one 

 system is far advanced, whilst others are but as yet only 

 indicated ; in another group we find this system in an early 

 stage of development, whilst others are far advanced, so that 

 it is impossible to offer a general definition of any of the 

 early ontogenetic stages, in terms of the degree of general 

 development. These varieties of degree of development affect 

 even the earliest stages ; in some of the Entomostraca histo- 

 genesis commences very early. Hacker (12) was able to re- 

 cognise the endo-mesomeres in the eight-celled stage in 

 (Jy clops; I have myself recognised them in the sixteen-celled 

 stage of an undescribed species allied to Ivc: and they were 

 recognised in the thirty-two celled stage in CetochUus by 

 Grobben (33) ; whilst, in many other Crustacea, histogenesis 

 is not apparent till much later stages. The condition of the 

 primitive shell-gland of Mollusca above referred to, is another 

 illustration of this point. 



It seems that, in the larval organs, we have the character 

 or set of characters, whose retention may be arbitrarily chosen 

 as the recognition-mark of the embryo as distinct from the 

 adult. Such a definition is unnatural in as far as it does not 

 define any particular stage of development, for, in some or- 

 ganisms, the larval organs are aborted very early, and, in 

 others, as already mentioned, not till potential adolescence, 

 yet it is no more artificial than any other definition based on 

 the degree of development of any other organ, and no general 

 definition is possible which would include all organs. This 

 definition is, on the other hand, a natural one, in one impor- 

 tant respect ; the retention of the larval organs is, in the vast 

 majorty of cases, attended by an environment or mode of life 

 differing markedly, and, in some cases, almost fundamentally, 

 from that of the organism after the loss of the organs in 

 question. In point of fact, this definition is in conformity 

 with common usage. 



