90 J. p. HILL. 



rapid growth complete tlie envelopment of the yolk-mass and 

 so constitute the lower hemisphere of the blastocyst. 



The bilaminar blastocyst of the Monotreme, formed in the 

 manner indicated above, is entirely comparable with the 

 Marsupial blastocyst of the same developmental stage. There 

 are differences in detail certainly (e.g. in the characters, 

 time of formation, and rate of spreading of the entoderm, 

 in the mode of formation of the blastocyst cavity and in its 

 contents, in the apparent absence in the Monotreme of any 

 well-marked line of division between the embryonal and extra- 

 embryonal regions of the ectoderm, in the relatively earlier 

 appearance of differentiation in the embryonal region in the 

 Monotreme as compared with the Marsupial), but the agree- 

 ments are obvious and fundamental ; in particular, I would 

 en^phasise the fact that in both the embryonal region is 

 superficial and freely exposed, and forms part of the blasto- 

 cyst wall just as that of the reptile forms part of the general 

 blastoderm. Moreover, should future observations confirm 

 the view of Semon that the primitive entodermal cells of the 

 Monotreme are proliferated off from the embryonal region of 

 the unilaminar blastoderm, then we should be justified in 

 directly comparing the latter with the unilaminar wall of the 

 Marsupial blastocyst, and in regarding it also as consisting 

 of two differentiated regions, viz. a formative or embryonal 

 region, overlying the white yolk-bed, and giving origin to 

 the embryonal ectoderm and the yolk-entoderm, and a non- 

 formative region Avhich rapidly overgrows the yolk-mass so 

 as to eventually completely enclose it, just as does the less 

 rapidly growing extra-embryonal ectoderm of the Saurop- 

 sidan blastoderm.^ Meantime I see no reason for doubting 

 that this rapidly growing peripheral portion of the unilaminar 

 blastoderm of the Monotreme is anything else than extra- 

 embryonal ectoderm homogenous with that of the reptile. 

 Indeed, I am not aware that any embryologist except Hubrecht 

 thinks otherwise. Even Assheton is, 1 believe, content to 



' We should further be jvistified in concluding that the entoderm is 

 similar in its mode of ont^in in all three mammalian sub-classes. 



