STUDIES IN MAMMALIAN EMBRYOLOGY. 551 



more or less in the opossum is nevertheless in favour of the 

 hypothesis of precocious segregation here brought forward. 



It remains doubtful whether it will be desirable to designate 

 the first-named spot as Selenka does by the name of " blasto- 

 pore.'^ An additional adjective will render good service^ and 

 the name '' cenogenetic blastopore " might recommend itself. 



Of the eight embryos in this stage which Selenka has 

 examined, five show this cenogenetic blastopore as a closed 

 proliferative spot ; in three of them small openings (Zellen- 

 liicken) were noticed^ in the wall of the blastocyst, which soon 

 closed up. In the hedgehog I have myself described {' Anat. 

 Anzeiger/ iii, pp. 511 and 907; 'Quart. Journ. Micr. Sci./ 

 1889, p. 286) a monodermic blastocyst, with hypoblast-cells 

 adhering to the wall at a spot which I think we are justified 

 in comparing to the opossum's cenogenetic blastopore. The 

 comparison of still earlier stages of the hedgehog is as yet 

 a desideratum in order to definitely sanction this comparison. 

 Here, too, the faint trace of the spot where the cenogenetic 

 hypoblast probably originates out of the wall of the mono- 

 dermic blastocyst is obliterated ; here, too, a didermic stage 

 follows with the two layers separate ; and here, too, the 

 connection between epi- and hypoblast which results from the 

 formation of the palingenetic blastopore is only a third phase 

 in this developmental process. 



For the mole we have Heape's observations, according to 

 which (1. c, pi. ix, figs. 17 — 27) the hypoblast arises much 

 in the same way as we have noticed in the shrew. It is hardly 

 possible to indicate a spot from whence the cenogenetic hypo- 

 blast can more particularly be seen to take its origin. And 

 the perforation which Heape indicates in his fig. 31 would 

 seem to me not to be homologous to the spot above noticed 



' Seleuka's figures of the blastocysts, m this and the followiug stages, are 

 drawn on very varied magnifying scales. This should be borne in mind in 

 comparing his figures with each other. It is apt to create some confusion, 

 from which in this paper I have endeavoured to keep clear by adhering very 

 strictly to the use of very few and always identical scales of enlargement for 

 the different figures. 



