The groove rapidly extends anteriorly and posteri- 

 orly, and at the same time becomes deeper and broader. When 

 it reaches its definitive position in the mid-ventral line, 

 it stretches throughout the entire length of the branchial 

 sac. 



It v/ill not be necessary to speak of the differen- 

 tiation of the endostyle into the various zones of cells 

 which go to make it up, as these have been described by nu- 

 merous authors: Delia Valle (3), Herdmann (7), Lahille (17), 

 and others. 



It is to be especially noticed in Figs. 10, 11, 17 

 and 21, that the rositions of tne pericardial rudiment and 

 endostylar groove in reference to each other remain the same 

 during the displacement of tne vesicle. Prom this fact it 

 is evident that the thick portion of tiie vesicle is carried 

 down bodily, and that no interstitial growth takes place in 

 this region during the process, else the distance betv/een the 

 pericardium and endostyle would not remain the same. It 

 cannot be said that the pericardial rudiment might compensate 

 by its own growth for any increase in extent of that part of 

 the wall against which it lies, for it covers practically 

 about the same area as long as it adheres to the vesicle. 

 It would seem, therefore, that the change in position of the 

 inner vesicle is brought about by the stretching out and 

 flattening of the cells in all but the thick area, and that 

 the latter is borne or rather pushed dov/n tov/ards the ventral 

 side . 



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