SEGMENTATION OF THE OVUM OF THE SHEEP. 229 



It suggests an explanation of the formation and origin of 

 the Eutherian amnion, which leads to the same conclusion 

 as Hubrecht^s view, namely, that the more primitive form of 

 amnion is probably that which is found in mammals like 

 Erinaceus, Cavia, or, as I think more truly, in Vespertilio, 

 and is of a diflPerent origin to the amnion of the Sauropsida 

 and the Mouotremata ; but it does not, on that account, demand 

 the removal of the placental mammals any further from those 

 groups than any other known anatomical differences do. 



Consideration of the Objections which may be urged to 

 THIS View of a Hypoblastic Origin for the Trophoblast. 



Tupaia. 



Hubrecht (33) in Tupaia javanica recognised an early 

 arrangement of some cells round others. He finds no differ- 

 ences during the first two generations, but when there are appa- 

 rently about a dozen cells or more he finds a differentiation into 

 an inner core of one lightly staining cell, which is completely 

 surrounded by an outer layer of darker cells. ^^Auf dieses 

 vierzellige Stadium folgt sehr rasch die Gruppirung der 

 weiteren Furchungsderivate, nicht urn eine centrale Hohluug, 

 sondern um eine centrale Zelle. Diese centrale Zelle fallt in 

 mehreren Schnitten durch ihre hellere Farbe auf, gleichsam 

 alsob ihr Protoplasma in etwas aaderer Weise auf das Tinc- 

 tionsmittel reagirt habe, wie dasjenige der peripheren Zellen.'^ 

 This rather later differentiation into two kinds of cells is very 

 like what I have described for the sheep, in which case I gave 

 some evidence to show that the differentiation occurred at the 

 third generation. 



According to Hubrecht, this differentiation marks embryo 

 from " trophoblast." The central cell, which soon multiplies, 

 gives rise to the whole of the epiblast and hypoblast, while the 

 outer layer is only " trophoblast,'^ and is destined to form no 

 part of the embryo proper. As appears in the latter part of 



