THE EARLY DEVELOl'iLEM OF THE MAUSUl'IALIA. /o 



sists ot' two legions distinct in origin and in destiny, viz. an 

 upper or formative region, derived from the upper cell-ring 

 of the lt>-celled stage, and destined to furnish the em- 

 brj'onal ectoderm and the entoderm and a lower or non- 

 formative region derived from the lower cell-ring of the 

 mentioned stage, and destined to form directly the extra- 

 embryonal or trophoblastic ectoderm (tropho-ectoderm) of the 

 bilaminar vesicle. 



(2) The formative region, unlike the non-formative, is 

 constituted by cells of two varieties, viz. : (i) a more 

 numerous series of larger, lighter-staining cells destined 

 to form the embryonal ectoderm, and (ii) a less numerous 

 series of smaller, more granular, and more deeply staining 

 cells, destined to give origin to the entoderm and hence 

 distino-uishable as the entodermal mother-cells. 



(3) The entodermal mother-cells, either without or subse- 

 quently to division, bodily migrate inwards from amongst the 

 larger cells of the unilaminar wall and so come to lie in 

 contact with the inner surface of the latter. They thus give 

 origin to the primitive entodermal ceils from which the 

 definitive entoderm arises. The larger passive cells, which 

 alone form the unilaminar wall after the inward migration of 

 the entodermal cells is completed, constitute the embryonal 

 ectoderm. 



(4) The entodermal cells as well before as after their 

 migration from the unilaminar wall are capable of exhibiting 

 amoeboid activity and of emitting pseudopodial processes, by 

 the anastomosing of which there is eventually formed a 

 cellular entodermal reticulum underlying, and at first co- 

 extensive with, the embryonal ectoderm. 



(5) The definitive entoderm thus owes its character as a 

 connected cell-layer primarily to the formation of secondaiy 

 anastomoses between the pseudopodial processes emitted by 

 the primitive entodermal cells (or entodermal mother- 

 cells). 



(6) The assumption by the entodermal cells of amoeboid 

 properties Avhilst they are still constituents of the unilaminar 



