384 PROCEEDINGS OF THE ACADEMY OF [April, 



of fig. 85. If we now examine section fig. 87, which is taken through 

 a vehger shghtly older than that shown in fig. 104, the relation of the 

 enteron to its surrounding structures may be ol^served. The large 

 entodermic cell, 4a^ has been successively traced through preceding 

 stages from its origin on the left side of the archenteron to its final 

 position on the right of the enteric cavity, as is shown in the figure. 

 Just posterior to this will be noted a mass of cells connecting the enteron 

 with the ectoderm. The nuclei of these cells are compact and deeply 

 staining, and the cytoplasm is decidedly clearer and contains less 

 yolk than that of the cells directly surrounding the enteric cavity. 

 Moreover, their position beside the large cell 4a- and now, through the 

 torsion which the enteron has undergone, their later position some- 

 what posterior to this cell, indicates the probability of their correspond- 

 ence with the "enteroblasts" of fig. 86 (PI. XXXI) and earlier stages, 

 in which the identity of these cells is unquestioned. 



It is proper in this place to consider again the results of Carazzi's 

 work on Aplysia and its relation to the mesentodermal history of 

 Fiona. It will be remembered that Carazzi's account of the hneage 

 of 4d up to a stage w'hen its derivatives number twelve cells exactly 

 parallels my results on Fiona, but regarding the fate of these cells 

 there is lack of agreement. The anterior small cells of Aplysia are 

 believed to be purely mesoblastic. while at least four of them in Fio7ia 

 appear, from the preceding account, to be entodermal in nature. 

 Carazzi, however, derives endoderm from the two small posteriorly 

 directed cells (e, e^ of Aplysia) which correspond to z\. z- of Fiona. 

 These latter cells were last seen lying at the posterior end of the gas- 

 trula of Fiona closely pressed against the ectoderm. At a later period, 

 when a large number of mesodermal elements lie in this region, the z^, 

 7? cells become indistinguishable from these. Sections of later stages 

 (fig. 87) show two cells which are larger and clearer than the entero- 

 blasts and which he against the ectoderm where the intestinal mass 

 touches it. They may represent the cells z^ z^, but of this there is 

 no evidence except that, given above. Anal cells are not a marked 

 feature of the developing embryo of Fiona, but at this time sections 

 in particular show two cells of somewhat larger size than the sm-round- 

 ing ectodermal elements, against which the forming intestine abuts 

 and which are doubtless comparable to the anal cells of other forms 

 (fig. 87, An.C). 



It will now be seen that the portion of the enteron lying most 

 posterior and close against the shell-gland invagination has been 

 derived from the cells which formed the bottom and the left side of 



