STUDIES IN MAMMALIAN EMBEYOLOGY. 501 



The chief questions about which the numerous authors 

 above cited disagree are — 



1. In what relation stands this archaic hypoblast of the 

 gastrula ridge to the remaining hypoblast of the didermic 

 blastocyst, which forms a closed layer, and in most mammals 

 even a closed sac beneath it? 



2. Is the production of mesoblast restricted to the tissue of 

 the gastrula ridge, or does the hypoblast proper take part in 

 such production ? 



3. Does the notochord originate solely out of a median 

 forward growth (Kopffortsatz) of the gastrula ridge (in which 

 a central canal may arise to a smaller or greater extent), which 

 pushes forwards between the primary layers, and which finally 

 fuses with the hypoblast (Einschaltungsstadien, Keibel), after 

 which this notochordal tissue again separates from the hypo- 

 blast .(Ausschaltungsstadien, id.), which then coalesces below 

 it to form the dorsal wall of the gut ? 



4. Or does a flat portion of the lower layer of the didermic 

 blastocyst participate in the formation of the anterior region 

 of the notochord in addition to this median rod of tissue 

 (Kopffortsatz) which has grown forwards out of the gastrula 

 ridge ? 



5. Do both these median structures, which we shall henceforth 

 designate as the protochordal wedge (median anterior pro- 

 longation of the gastrula ridge, often enclosing a protochordal 

 canal or rudiments of it) and as the protochordal plate 

 (developed in situ as part of the hypoblast), take part in the 

 formation of lateral paired plates of mesoblast? 



6. Does the gastrula ridge and its anterior prolongation 

 (the protochordal wedge), together with the longitudinal 

 cavity therein contained, represent the whole hypoblast and 

 the archenteric cavity of Ampliioxus, so that the inner layer 

 of the didermic blastocyst is not the homologue of one of 

 the germinal layers, but simply a yolk-membrane (para- 

 derm, Kupff'er ; lecitophor, van Beneden), peculiar to the 

 Amniota ? 



It will be clear from the enumeration of these points of 



