DEVELOPMENT OF THE ALBINO RAT 107 



respective figures, these forming the region of the future anterior 

 ends of the respective embryos. In C of figure 32 is shown a 

 drawing of one of the sections of a series of cross-sections of an 

 egg-cylinder taken from rat No. 41, 8 days, 16 hours, after in- 

 semination, presenting a stage in which the primitive groove 

 may be definitely made out. This figure is not unlike figure 6 

 of the article of Widakowich, obtained from an egg-cylinder 

 secured on the eighth day. Concerning this figure he state-: 

 "Das Ectoderm steht in direktem Zusammenhange mit zwei 

 Mesodermzungen die gegen die der Primitivrinne gegenuberlie- 

 gende Seite zu auswachsen." The section drawn in C of this 

 figure is taken from the region very near the junction of the 

 primary embryonic and the extraembryonic ectoderm, as may 

 be observed from the character of the entodermal cells, in the 

 lower part of the figure. The increase in the thickness of the 

 mesodermal wings, the result, in part at least, of proliferation 

 of mesodermal cells, as evidenced by the presence of mitotic 

 figures, is clearly shown in this figure. The mesoderm is dis- 

 tinctly separable from the entoderm as also from the primary 

 embryonic ectoderm except in the region of the primitive streak 

 and groove. The growth of the mesoderm after its anlage has 

 been correctly shown for the albino rat by Selenka, Robinson, 

 and Widakowich; the latter especially giving excellent figures. 

 His figure 5 is especially instructive. In this, he represents the 

 appearances shown by two views of an isolated egg-cylinder, with 

 the primitive groove in anlage, showing the lateral extensions 

 of the mesoderm. Sobotta ('11) has given the best and most 

 comprehensive account of the anlage and growth of the meso- 

 derm in the mouse. An excellent cross-section of a mouse egg- 

 cylinder in the primitive streak stage is presented in his figure 

 6, which presents very similar appearances to my C of figure 32. 

 None of the figures of cross-sections of egg-cylinders included 

 by me show the very beginning of the anlage of the mesoderm, 

 though A of figure 32 approaches this very closely, as does also 

 figure 31, presenting a sagittal section. The evidence at hand 

 warrants the conclusion that in the albino rat, the mesoderm 

 has its anlage in the caudal region of the primary embryonic 



