oytial nasr of tissue left i'^ the re^-tre r^f i]-xe e-hryo 

 after the eetoderr has been forred ard nerarated off by 

 the develo-nert of the -pGr^loea. The irrer perr layer 

 as a rule is for~ed -uch later thar, the ectcderr. Soon 

 after the simcrtinp '-e'^brrre is developed cell bourdaries 

 be.fin to a-^r^ear in the syneytiun in the i-^terior of the larva. 

 The cells thus forned are ririritive endcderral cells, and 

 are crowded together without any defirite arranre^ent for 

 a nurber of hours. Stares ir. -hich the cell vrallr. are rean- 

 r>earinp are shown ir Figures 34 to 36, V.hen the enbryo 

 is about forty-eirht to sixty he\jrc old, the tlr-e at which 

 attachment takes nlace, a fissure a^'^ears i" the -^iddle 

 of the ^ass of endoder'-al tissue. This is the befinninp 

 of the ccelenteric cavity. This separation begins near 

 the anterior nart and rrcws toward the rosterior end. The 

 coelenteron f^rndually increases in size, and at the sane 

 tine the e'-doderrai cells b'-pin tc be rearranged; and fin- 

 ally becore situated narallel to each other with their baser 



