STUDIES IN MAMMALIAN EMBEYOLOGY. 545 



2. The Protochordal Plate, Protochordal Wedge, and Annular 

 Zone of Modified Hypoblast in other Mammals, according 

 to Bonnet, van Beneden, Selenka, Fleischmann, a. o. 



In Bonnet's description of the developmental phenomena of 

 the sheep very numerous points of coincidence with what has 

 here been described for the shrew have already been repeatedly 

 pointed out. Bonnet emphatically insists on the participation 

 of an anterior (ab origine hypoblastic) chorda-entoblast 

 towards the formation of notochord and mesoblast, in which 

 further backwards the '^ Kopffortsatz" (our protochordal 

 wedge) and gastrula ridge participate. His statements are 

 all the more valuable as in his later paper (1. c, 1889) he 

 recognises (p. 75) that the different results at which other 

 authors have arrived have led him to most careful and repeated 

 reperusal of his series of sections, and have changed his mind 

 in this sense, that in a former publication (1. c, 1884) he gave 

 too prominent a place to the anterior hypoblastic plate in the 

 formation of the notochord. He is now willing to accord a 

 much more considerable part to the '' Kopffortsatz/' of which 

 he had formerly underrated the length ; but he holds on as 

 strongly as ever to the participation of a purely hypoblastic 

 anterior portion. It will be best to quote his own words, 

 which are nearly verbally applicable to the shrew. He says 

 (I.e., 1889, p. 84): 



" The definite formation of the notochord is only brought 

 about when the gutter-like or flat 'Kopffortsatz' that has become 

 intercalated in the entoblast is again pinched off longitudi- 

 nally ; only now we may, rigidly speaking, apply the terms 

 ' notochord ' and ' notochordal lumen,' in so far as by this 

 latter name one would wish to designate remains of the folded- 

 off enteric cavity in the segregated notochord. In the genesis 

 of the notochord we must thus very strictly keep apart the ori- 

 ginally solid but subsequently canalised Kopffortsatz, its ventral 

 fusion by which it opens and becomes gutter- or plate-shaped, 

 and intimately connected with the enteric hypoblast. Poste- 

 riorly the ' Kopffortsatz ' passes into the gastrula ridge, 



