550 A. A. W. HUBEECHT. 



In the opossum, Selenka (1. c. p. 114) describes and figures 

 a very early blastula stage in which one or a few cells get 

 separated from the outer blastula wall, wander into the archi- 

 coel (segmentation cavity), and form the starting-point for 

 the development of the hypoblastic sac. In the beginning 

 (Selenka's figs. 3, 8, and 11, pi. xvii; 2, 3, and 4, pi. xviii) 

 these rapidly multiplying cells adhere to the outer blastula wall 

 in a spot which Selenka calls the blastopore. When the for- 

 mation of the hypoblastic sac is completed there is a stage in 

 the development of the opossum's didermic blastocyst when 

 the two layers are no longer thus primarily con- 

 nected, but are separate though contiguous spherical 

 sacs, the hypoblast consisting of flattened, the epiblast of more 

 massive cells (Selenka's figs. 2, 3, 6, pi. xix). 



Only in a further stage — immediately following on the fore- 

 going — the palingenetic hypoblast (gastrula ridge and proto- 

 chordal wedge) makes its appearance; the fusion which 

 in consequence of this is established between epiblast and 

 hypoblast stands in no direct relation to the earlier con- 

 nection above alluded to, and which is noticed in the blastula 

 stage. 



There can thus be no doubt that the wall of the monodermic 

 blastocyst of the opossum produces cell-material, which arranges 

 itself into a second closed sac inside the primary one. The 

 spot from whence this cell production emanates is distinct 

 and circumscribed, it disappears without leaving any trace 

 when the didermic blastocyst is fully established. Shortly 

 afterwards there appears in about the same region of the 

 outer layer a renewed cell proliferation which leads to the 

 formation of the gastrula ridge. 



The latter is what I have proposed to call the palingenetic, 

 the former the cenogenetic hypoblast. In the opossum the 

 clearness of the phenomenon leaves nothing to be desired. It 

 is not a priori necessary that the spot from whence the ceno- 

 genetic hypoblast originates should be the same as that where 

 afterwards the palingenetic blastopore (gastrula ridge, primi- 

 tive streak) makes its appearance. That it appears to do so 



