372 H. H. Newman and J. T. Patterson. 



spending stages of the rat and the mouse, described respectively 

 by Selenka, '84, fig. 29, Taf . XIV., and Melissinos, '07, figs. 38 

 and 39 Taf. XXXIV. He thus finds the vesicle composed of 

 three sacs lying one within the other: the innermost one is the 

 ectoderm, the middle the entoderm, and the outer the tropho- 

 blast (hinfalJigen Ectoderm), which at the proximal or attached 

 end of the vesicle is differentiating into the Trager. The simil- 

 arity between the vesicle of Fernandez and those figured by Melis- 

 sinos (his figs. 38 and 39) is particularly striking, though, as he 

 points out, there are several differences. In the first place, the 

 mesoderm is not yet formed and the so-called Trager cavity scarce- 

 ly can be regarded as homologous with that of the mouse. In the 

 second place, the parietal layer of the yolk-sac entoderm is not 

 complete, but is wanting in the distal portion of the trophoblast. 

 If, however, we may be allowed to make a suggestion based on a 

 study of his photograph (fig. 6, Taf. XIX), what appear to be 

 scattering cells lying along the inner surface of the distal tropho- 

 blast might well be interpreted as representing the remains of the 

 parietal layer of the yolk-sac. This would make this early stage 

 of the Mulita very closely resemble the corresponding stages of 

 several other forms, as illustrated in the figures of such investi- 

 gators as Selenka ('84), Robinson ('92), Jenkinson ('00), and 

 Mellissinos ('07). 



The most interesting portion of this young vesicle of the Mulita 

 is the inner sac, for it is the primordium out of which the ecto- 

 derm of the several embryos later differentia tes. Fernandez 

 points out the significant fact that it gives no indication of being 

 a multiple structure, such as one would expect to see if the vesicle 

 were the product of the fusion of several eggs. 



The second stage of Fernandez is decidedly more advanced than 

 the preceding, and was found lying loose in the fund us end of 

 the uterus. In the preserved condition it measured 3 mm. long 

 by 2.3-2.5 mm. wide. The general condition of the germ layers 

 in this vesicle is made clear in the slightly modified copy of his 

 second text-figure (fig. 2). The figure, which is a diagram of a 

 median longitudinal section passing through two embryos, is 

 shaped like a horse shoe. The entire convex anterior and lateral 



