IV. i INITIAL STRUCTURE OF THE GERM 181 



undifferentiated material. Morgan has attempted to answer 

 this question by counting the number of cells in the various 

 organs of whole and partial larvae. It appears from his estimate 

 that the number of cells in the archenteron, in the notochord 

 and in the nerve-cord of y, ^, and ^ larvae is constant, and that 

 though the size of the whole body is less, the dimensions of the 

 notochord and nerve-cord are about the same in all three. It 

 would seem, therefore, that there is a minimum size and a 

 minimum number of cells necessary for the formation of these 

 organs. Whether the failure of the f blastomere to develop 

 into a larva is, however, due to mere lack of material or the 

 absence of some specific substance needful for the development 

 of some particular organ is still undecided ; for the -| and cells 

 contain substance from both the animal and vegetative portions of 

 the egg, while the f are composed of one or the other substance 

 exclusively, the furrows of the third phase being equatorial. 



LITERATURE 



T. H. MORGAN. The number of cells in larvae from isolated blasto- 

 meres of Amphioxus, Arch. Ent. Mech. iii, 1896. 



E. B. WILSON. Amphioxus and the Mosaic theory of development, 

 Jown. Morph. viii, 1893. 



& 5. COELENTERATA. 







Many years ago Metschnikoff observed a perfectly normal 

 separation of the blastomeres at a certain period in the develop- 

 ment of certain medusae (Oceania). If when the cells reunite 

 in a later stage the order of their rearrangement is not constant, 

 the egg-substance must be in some measure isotropic. 1 



More recently the equipotentiality of the blastomeres of these 

 forms has been experimentally demonstrated by Zoja (Fig. 91). 



The cells were separated by means of a needle. \ and \ 

 blastomeres (in the case of Liriope, Geryonia, Mitrocoma, Clytia, 

 and Laodice) and f and $ blastomeres in the case of the two 

 last mentioned will give rise by normal segmentation to blas- 



1 It should not be forgotten that in 1869 Haeckel had cut the blastulae 

 of Crystallodes (a Siphonophor) in pieces and obtained from the larger 

 fragments normal larvae. The development of small pieces was re- 

 tarded and abnormal (Zur Entwickelunysgeschichte tier Siphonophortn, 

 Utrecht, 1869). 



