Minot.] 168 April 18, 



meriting as a whole, but remaining as one large continuous mass, 

 which, according to the recent observations of Ulianin and Ray 

 Lankester, forms part of the wall of the alimentary canal, the rest of 

 which arises from the yolk by the separation of small cells. The 

 same thing occurs among Vertebrates (e. g., fishes and birds), while 

 in others, as among the Annelids, the yolk divides (e. g., Batrachia, 

 Mammalia). In the former case, as for instance in fishes, the so- 

 called embryonic disc is the ectoderm, the yolk the entoderm, while 

 that which lies between is not hypoblast, as it is usually called, but 

 mesoblast. When the infolding of the embryonic rim (cf. His, Bal- 

 four and Oellacher) begins the formation of the chorda dorsalis and 

 of the nervous system, then small cells separate from the yolk and 

 form underneath the axis the upper wall of the primitive gut, while 

 the yolk itself makes the lower wall (cf. Balfour, on the early stages 

 of Vertebrates, Journ. Anat. Physiol., x). Ranber has made it very 

 probable that the same form of axial growth which occurs in Teleosts 

 and Selachians reoccurs in the chick, and probably in all Verte- 

 brates. It can be shown that it really is so in frogs. It is this mode 

 of axial infolding l which causes the so-called bigeminal evolution of 

 Vertebrates, certainly at least in the lower forms. The same phe- 

 nomenon has been traced in some Annelids (leeches, growing anal 

 end of naids, and in the budding zones of some Oligochaeta). It may 

 be considered probable that this mode of embryonic development 

 will be found to be the most important and general characteristic of 

 segmented animals. Among all the chordate animals the difference 

 between the yolk and formative substance (Bildungsdotter) has been 

 long known, and the collection of the finely granular matter at the 

 pole where the small ectodermic cells are first segmented off has been 

 frequently pointed out. In other cases the development is not polar 

 but by delamination. Even in these instances the ectodermic cells 

 are smaller than the others (vide BischofF, Kolliker, Hensen, and fid. 

 van Beneden, on mammals, Strieker, Bambeke and Goette on batra- 

 chians). It must, however, be borne in mind that we are still far off 

 from understanding the development of the mammalia because the 

 origin of the chorda dorsalis has never been determined. 



Concerning the segmentation of the eggs of Arthropods so little 

 is known, that nothing further can be affirmed than that the outer 



1 Semper in his memoir on " Strobilation and Segmentation," (Semper's Arrei- 

 ten, Bd. iii) calls the axial infolding " Verwachsung der beiden Keimstreifhalften." 



