THE ENERGY OF DIVISIOX 



291 



annelids and gasteropods, for example, the entire ectoblast arises 

 from twelve micromeres segmented off in three successive quartets 

 of micromeres from the blastomeres of the four-cell stao"e. In 

 Echinus, according to Morgan, the number of cells used in the forma- 

 tion of the archenteron is approximately one hundred ; in Spluzrechinus 

 the number is approximately fifty. 



Perhaps the most interesting numerical relation of this kind are 

 those recently discovered in the division of teloblasts, where the num- 

 ber of divisions is directly correlated with the number of segments or 

 somites. It is well known that this is the case in certain plants 

 {C/iaracca), where the alternating nodes and internodes of the stem 

 are derived from corresponding single cells successively segmented 

 off from the apical cell. Vejdovsky's observations on the annelid 

 Dc7idrob(zna give strong ground to believe that the number of meta- 

 merically repeated parts of this animal, and probably of other anne- 

 lids, corresponds in like manner with that of the number of cells 

 segmented off from the teloblasts. The most remarkable and 

 accurately determined case of this kind is that of the isopod Crustacea, 

 where the number of somites is limited and perfectly constant. In the 

 embryos of these animals there are two groups of teloblasts near the 

 hinder end of the embryo, viz. an inner group of mesoblasts, from which 

 arise the mesoblast-bands, and an outer group of ectoblasts, from 

 which arise the neural plates and the ventral ectoblast. McMurrich 

 ('95) has recently demonstrated that the mesoblasts always divide 

 exactly sixteen times, the ectoblasts thirty-two (or thirty-three) times, 

 before relinquishing their teloblastic mode of division and breaking 

 up into smaller cells. Now the sixteen groups of cells thus formed 

 give rise to the sixteen respective somites of the post-naupliar region 

 of the embryo (/.r. from the second maxilla backward). In other 

 words, each single division of the mesoblasts and each double division 

 of the ectoblasts splits off the material for a single somite! The 

 number of these divisions, and hence of the corresponding somites, 

 is a fixed inheritance of the species. 



The causes that determine the rhythm of division, and thus finally 

 establish the adult equilibrium, are but vaguely comprehended. The 

 ultimate causes must of course lie in the inherited constitution of the 

 organism, and are referable in the last analysis to the structure of 

 the germ-cells. Every division must, however, be the response of the 

 cell to a particular set of conditions or stimuli ; and it is through the 

 investigation of these stimuli that we may hope to penetrate further 

 into the nature of development. It must be confessed that the 

 specific causes that incite or inhibit cell-division are scarcely known. 

 The egg-cell is in most cases stimulated to divide by the entrance of 

 the spermatozoon, but in parthenogenesis exactly the same result is 



