10 J. r. HILL. 



rently did not regard tlie " urentodermzelle " as tlie sole 

 progenitor of the entoderm, but simply as an entoderni-cell 

 precociously inturned from the " blastoporic " margin. 



This view of Selenka, however, lauds us in the predicament 

 of having to regard the embryonal area as differentiating over 

 the vegetative hemisphere, since in the nest stage the 

 '^blastopore'' is described as beiug situated excentrically in 

 that area. Either Selenka's determination of the poles in the 

 42-celled blastocyst is wrong, or the entoderm does not 

 originate as he describes it. My own observations force me 

 to accept the latter alternative. In his paper Selenka gets 

 over the difficulty very easily by altering the orientation of 

 his figures. On Taf. xvii, the figures of sections of blasto- 

 cysts are so placed that the '' blastopore " is below, next the 

 bottom of the plate. These figures I hold to be correctly 

 orientated. On Taf. xviii, the figures are inverted, so that 

 the '^blastopore " is above; as the result the animal pole of 

 fig. 11, Taf. xvii, becomes the vegetative pole of the stage 

 next described (fig. 2, Taf. xviii).] 



The stage just referred to, described as an "eiformige 

 gastrula," is represented in a drawing made from the fresh 

 specimen as lying quite free in a large perivitelline space 

 enclosed by a very thick layer of albumen, outside which is 

 the " granulosa-membran." In section (fig. 2) a mass of 

 entoderm is seen to reach the surface at one pole (marked hi.) 

 uppermost in the figure, Avhilst other entodermal cells are 

 shown spreading from this towards the lower pole. The 

 ectoderm of the wall is represented as composed of definitely 

 cubical cells. [The presence of a large perivitelline space, 

 by itself stamps this specimen as not normal. The sectional 

 figure nmst be schematic] 



The last of Selenka's early stages to which reference need 

 be made here is formed by eight " gastrulte" (blastocysts), 

 reckoned as ten hours after the commencement of cleavage 

 [a reckoning I consider of no value] (Taf. xviii, figs. 3 and 4). 

 The embryonal area is now distinguishable by the larger size 

 of its ectodermal cells. The entoderm is unilaminar, and has 



