104 G. CARL HUBER 



consideration of older stages, and the making of a number of 

 reconstructions, which it is not contemplated to consider now. 

 It must suffice to state at this time that in the albino rat, as 

 shown by Widakowich and here shown by me, it is possible to 

 delineate clearly the primary embryonic ectoderm and to show 

 that the first evidence of the mesoderm is found antimesometrial 

 to the future amniotic fold and in the region of the future primi- 

 tive streak; therefore is mesoderm which I would regard as 

 peristomal mesoderm in the sense of C. Rabl, reference to which 

 is made by Sobotta in his discussion of this question. It may 

 be that the rat offers more suitable material for the elucidation 

 of this question than is to be found in the mouse. In the albino 

 rat, the anlage of the mesoderm is from the sagittal portion of 

 the caudal region of the primary embryonic ectoderm, the caudal 

 part of the future primitive streak and antimesometrial to the 

 amniotic fold. Sobotta gives very favorable consideration to 

 the observations of Widakowich, touching this question, which 

 he regards as "Bei weitem die beste Darstellung des Gegenstan- 

 des." My own observations fully confirm those of "Widakowich. 

 These questions will receive fuller consideration in a later pub- 

 lication dealing with the embryology of the albino rat, carrying 

 the development from the time of the anlage of the amniotic fold 

 to the stage of embryo form, the material for which is at hand. 



In figure 32 are shown cross-sections of the antimesometrial 

 portion of three egg-cylinders in the region of the developing 

 mesoderm. Sections drawn in A and B, were taken respectively 

 from egg-cylinders obtained from the same uterus as was the 

 one shown in sagittal section in figure 31, rat No. 34, 8 days, 17 

 hours, after insemination; C, from rat Xo. 41, 8 da}'s, 16 hours, 

 after insemination. It is very probable that the series from 

 which A of this figure was drawn, is net cut in exactly the cross 

 plane. A study of the series shows, however, that the deviation 

 from this plane is not marked. The sections from which this 

 figure was drawn pass a little below (antimesometrial) to the 

 region of junction of the primary embryonic and extraembry- 

 onic ectoderm. To one side, the lower in the figure, the primary 

 embryonic ectoderm shows a slight thickening and evidence of 



