62 J. P. HILL. 



latter and to its sister-cell. It is, in fact, a primitive ento- 

 dermal cell, as comparison with fif^. 68 proves, and that it has 

 been formed by the division of a mother-cell situated in the 

 nnilaminar wall can hardly, I think, be doubted. Its sister- 

 cell, which is still a constituent of the wall, would presumably 

 have migrated inwards some time later. 



It is to be noted that the primitive entodermal cell refei'red 

 to above and those depicted in figs. 71 and 72 are definitely 

 contoured, ovalish and rounded cells, entirely devoid of pro- 

 cesses. In these respects they differ markedly from the ento- 

 dermal cells shown in figs. 68-70, which are very variable in 

 form owing to their possession of more or less elongated 

 psendopodial-like processes. It might thei'efore be inferred 

 that the formation of these processes only takes place after 

 the entodermal cells have become definitely internal. Such 

 an inference, however, would be incorrect, for I have abundant 

 evidence showing that such processes may be given off from 

 the entodermal mother- cells whilst they are still constituents 

 of the wall. In in toto preparations, it is often difficult to 

 determine with certainty whether a particular entodermal cell 

 still enters into the constitution of the unilaminar wall or not. 

 In the portion of the formative region of a '04 vesicle depicted 

 in fio-. 70, however, I am satisfied that all the entodermal 

 cells therein shown (they are readily distinguishable by theii* 

 smaller size and more deeply staining character) are, with the 

 possible exception of the one on the extreme right, at least 

 partially intercalated between the larger ectodermal cells of 

 the wall. Some of them are entirely situated in the wall; 

 others have extended inwards in varying degree so as to 

 partially underlie the ectodermal cells. It is these latter 

 entodermal cells in particular which exhibit the cytoplasmic 

 processes above referred to. As the figure shows, these pro- 

 cesses have all the characters of pseudopodia; they vary in 

 size, form, and number from cell to cell, individual processes 

 may be reticulate and their finer prolongations may anasto- 

 mose with those of others, and they are formed of cytoplasm, 

 less dense and rather less deeply staining than that of the 



