532 A. A. W. HUBRECHT. 



rated and retarded. Supposing for a moment the palingenetic 

 phenomena in the region of the primitive streak were to follow 

 the type of the Reptilia, a wide-open blastopore ensuing, and 

 now this palingenetic hypoblast to fuse with the pre-existent 

 cenogenetic hypoblast underlying it, and to turn into a gas- 

 trula in the way that was indicated above (p. 521), the fatal 

 effect would inevitably be that the fluid contents would escape, 

 and that the blastocyst would collapse. A first safeguard 

 against this danger is the fact that the hypoblast is a closed 

 sac; a second that the canal in the protochordal wedge — a 

 palingenetic remnant of the posterior medio-dorsal region of 

 the archenteron — is (1) often absent, and that (2) when present 

 it is exceedingly fine, capillary resistance thus counteracting 

 the tendency of the enclosed fluid to escape by that canal. 



Moreover the canal is, firstly, very much bent forwards 

 under nearly right angles to the radius of the blastosphere, 

 which is another physical impediment towards the escape of 

 fluid ; secondly, only in later stages, when the so-called " in- 

 tercalation ^' in the hypoblast has come about, it is in a more 

 or less extensive communication with the cavity of the yolk- 

 sac ; thirdly, the attachment of the blastocyst within the 

 uterine cavity has by that time become more definite, and 

 thereby the pressure above the germinal area more or less equal 

 to that inside the blastocyst.^ 



* There is a figure in Selenka's treatise on the Opossum (27, pi. xviii, 

 fig. 3) which at first sight would seem to go dead against the hypothesis here 

 developed, because it shows a small pore in a blastocyst at an early though 

 already considerably expanded stage. It should be noted — 1. That the 

 possible escape of fluid contents may in this case be most effectually coun- 

 teracted by the thick albuniiuiferous layer enveloping the blastocyst. 2. That 

 in other similar stages (1. c., figs. 4 and 10) there is no trace of a similar 

 opening. So that I think my suggestion also holds good for the opossum. 

 The phenomenon of stretching of the blastocyst wall, without increase in the 

 number of cells, is very marked in Selenka's figures (cf. 1. c, pi. xviii, figs. 2 

 and 3). 



A stage directly comparable to the one just noticed is figured by Keibel (17, 

 1889, pi. xxiv, figs. 46(5 and 47) for the rabbit, and by Heape (9, pi. viii, 

 fig. 31) for the mole. In both cases it cannot be said to be an actual perfora- 

 tion. In all the three cases it is in the region just behind the embryouic shield 



