60 J. r. HILL. 



there are preseut, in close apposition with the inner surface of 

 the uuilaminar wall, small, darkly staining cells, apparently 

 quite isolated from each other and usually of flattened form 

 (figs. 73, 74, 76, eat.). One has only to glance at a well- 

 stained in toto prepai'ation of the formative region (cf. 

 fig. 70) to realise how inadequate such a descrijjtion of the 

 primitive entoderm cells really is. 



Sections nevertheless do yield valuable information on 

 certain points. Besides affording the negative evidence of 

 the absence of tangential divisions and the positive evidence 

 that the primitive entodermal cells are actually internal (figs. 

 73, 74, 76), they show that growth of the wall in tliickness 

 has already set in, and that it is most marked over the 

 formative region, though the thickness attained by the cells 

 is as yet very unequal (tigs. 73-76). Measurements taken 

 from an '04 vesicle show that over the non-formative region 

 (fig. 77) the cells vary in thickness from "006 to "009 mm., 

 whilst over the formative region the range of variation is 

 greater, viz. from "006 to "Olo mm., so that we may conclude 

 that the latter region is on the average thicker than the 

 former (cf. figs. 73-76, with fig. 77 depicting a small portion 

 of the non-formative region). It is still impossible to deter- 

 mine the position of the sutural line, even in sections of 

 fragments of the wall known to contain it. 



The entodermal mother-cells are not very readily recog- 

 nisable in sections. In fig. 75, however, which is drawn 

 from an accurately transverse section through the formative 

 region of an '04 vesicle, there is depicted what is undoubtedly 

 an entodermal mother-cell {ent.). The interesting point 

 about this particular cell is that its cell-body, whilst still 

 intercalated between the adjoining cells of the uuilaminar 

 wall, has extended inwards so as to directly underlie one of 

 the wall-cells. Division of such a cell as this would neces- 

 sarily result in the production of an internally situated cell 

 with all the relations of one of the primitive entodermal type. 

 The inwardly projecting spheroidal cell situated immediately 

 to the left (in the figure) of the one just referred to, I also 



