STUDIES IN MAMMALIAN EMBRYOLOGY. 513 



subjacent to but independent from the epiblastic shield) and a 

 posterior swollen knob of this epiblastic shield. This fusion is 

 at the early stage so superficial that it is as yet easy to distinguish 

 between the proliferated epiblastic cells of the gastrula ridge 

 and the flattened hypoblast-cells that adhere to them (figs. 37 

 and 38). Moreover^ the fusion at first only affects half a dozen 

 hypoblast-cells. Certain important data should here be borne in 

 mind as following from stage 73/, viz. (a) that the first gas- 

 trula-ridge proliferation arises at the posterior end of the 

 epiblastic shield, and not anywhere towards its middle ; (6) that 

 the first indication of a forward growth (protochordal wedge) of 

 this proliferation (fig. 39. p. w.) is unmistakably present; (c) 

 that the epiblast in front of the gastrula ridge is henceforth 

 quite distinct from that which belongs to the ridge ; {d) that a 

 perforation of the epiblast in the anterior end of the gastrula 

 ridge can be faintly noticed (figs. 38 and S9p), it being very 

 questionable whether it actually opens out at both ends. Also 

 later on the traces of a lumen (canal) in the protochordal 

 wedge (Kopffbrtsatz) are so sporadic (figs. 50, 69, 73, 78, 79, 

 81, 85, 86, 90) that I would not venture to apply the diagram 

 which Heape has given for the mole's neurenteric canal without 

 restriction to the shrew. 



The point where this fusion between the originally inde- 

 pendent sheets of epiblast and hypoblast first comes about is no 

 doubt the spot that also passes by the name of Hensen^s knob. 

 Forward from it there is a growth that gives rise to uotochord 

 (pro parte) and gastral mesoblast (Rabl) ; posteriorly we 

 find the region of the peristomal mesoblast. Such peristomal 

 mesoblast is as yet absent in the embryo 73/; to its formation 

 the further backward growth of the proliferation is pre- 

 liminary. The gastrula ridge gradually stretches backwards. 

 Posteriorly it again dilates into a caudal swelling (Schwanz- 

 knoten, Endwulst; figs. 34, 35, 56, 59, 62, 63). There is 

 here a very sudden passage from the epiblast of the embryonic 

 shield, which is involved in the formation of this caudal swell- 

 ing of the gastrula ridge into the epiblast outside of the 

 embryonic shield (cf. fig. 56 and fig. 59). 



