88 J. r. HILL. 



cellukr lens-shaped blastoderm of the preceding stage has 

 now extended iu the peripheral direction so as to enclose 

 about the upper half of the yolk-mass, and. in so doing it has 

 assumed the form, almost exclusively, of a unilaminar thin 

 cell-membrane, composed of flattened cells and closely applied 

 to the inner surface of the zona. At the embryonic pole, 

 however, in the region of the white yolk-bed, there are 

 present in the Ornithorhynchus egg a few plump cells, 

 immediately subjacent to the unilaminar blastoderm, but 

 sepai-ate and distinct from it, whilst in the Echidna egg 

 Semon's figure (fig. 33), which is perhaps somewhat schematic, 

 shows a gi'oup of scattered, cells, similar to those in the 

 Ornithorhynchus egg but placed considerably deeper in the 

 white yolk-bed. Unfortunately we have no definite evidence 

 as to the significance of these internally situated cells. One 

 of two possible interpretations may be assigned to them. 

 Either tliey represent the last remaining deeply placed cells 

 of the blastodisc of the preceding stage, which have not yet 

 become intercalated in the unilaminar blastodermic membrane 

 believed by Semon to be the condition attained in eggs of 

 about this stage of development, or they are cells which have 

 been proliferated off from this unilaminar bhistoderm, to 

 constitute the parent cells of the future yolk-entoderm. As 

 regards Echidnn, Semon expresses a definite enough opinion ; 

 he holds that these deeply placed cells actually arise by a 

 somewhat diffuse proliferation or ingi'owth from a localised 

 depressed area of the blastoderm at the embryonic pole, and 

 that they give origin to yolk-entoderm. This interpretation 

 of Semon seems probable enough in view of the mode of origin 

 of the entoderm in the Metatheria and Eutheria. Moreover 

 in the next available stage, an egg of Ornithorhynchus, just 

 over 6 mm. in diameter, described by Wilson and Hill, the 

 blastoderm is already bilaminar throughout its extent, so that 

 •we might very well expect to find the beginnings of the ento- 

 derm in the somewhat younger eggs. 



In the G mm. egg just referred to, the peripheral portion of 

 the unilaminar blastoderm of the preceding stage has grown 



