524 T. IKEDA : 



is eeeentrically jJnced nearer to tlie niiterior end, has to traverse 

 a longer distance l)efoi'e it can l)e reflected inwards, and thns on 

 its inward conrse lags beliind the anterioi- and antei'o-lateral por- 

 tions. Eventually all the cells of the wall of the groove that are 

 left behind after proliferating the mesohlast cells, are without 

 doul)t invaginated and forai a part of tlie ent<)1)last. The groove 

 then entii-ely disappears. I could not discover any remnant of it 

 in any pai't of the posterior region where, according to Cald- 

 well, the ectoblast and the entoldast are said to stand in fusion 

 to give rise afterwards to the aims. In such an advanced stage, 

 the antei'ior diverticula have also ceased to give off mesohlast 

 cells and have become straightened out, their walls acquiring a 

 normal epithelial character (entoblastic). 



From the facts above adduced, it may 1)e concluded that 

 hoik the anterior <liverticiiJn and the ventral groove, présent at <i 

 cerkitn developmental Hage of the Phoronis embryo-'^, are remnanti^ of 

 tlie original mesentohlast ivhich at an. earlier dage occnpied the 

 the whole extent of the gaxlral invagination. TJicy are, therefore, 

 merely temporary, and destined, sooner or later to split into meso- 

 blastic and entoblastic cells. 



As will be seen in figs. oO a-e, the ectoblast and the archen- 

 teric walls are l)rought together into such close contact, especially 

 along the dorsal and lateral regions that scarcely any interspace 

 is left between them. In the embryo given in fig. 8, the cavity 

 of the rudimentary preoral lobe is filled with mesohlast cells pro- 

 duced from the original mesentoblastic layer. So far as I can make 

 out, these show no difference whatever from those proliferated 

 from the anterior diverticula : both are indistingm,shably mixed 

 together. Though most of the mesohlast cells in the preoral lobe 

 lie loose during the active period of the diverticula, there are 



