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Prof. J. T. Wilson and Mr. J. P. Hill. 



[Jan. 21 



" Primitive Knot and Early Gastrulation Cavity co-existing with 

 Independent Primitive Streak in Ornithorhynchus." By 

 Professor J. T. Wilson, M.D., and J. P. Hill, D.Sc., Univer- 

 sity of Sydney. Communicated by Professor G-. B. Howes 

 F.E.S. Eeceived January 21 — Eead February 12, 1903. 



Amongst the material at our disposal for the investigation of the 

 earlier stages in the development of the Monotremes is an egg of 

 Ornithorhynchus, measuring 10 x 9*5 mm. The stage of development 

 represented by this egg is of such interest and importance that we 

 have deemed it deserving of a brief preliminary communication. 



The stage of embryonic organisation would appear to fit in as 

 immediately succeeding the oldest of the early embryos described by 

 Semon,* viz., his "E G " (fig. 15, Taf. 8). His "E r " appears, indeed, 

 to have been older than " E 6 ," but with the exception of one figure 

 (fig. 39, Taf. 9) representing the structure of the extra-embryonic 

 blastoderm, Semon gives no indication of the conditions met with in 

 that ovum. This is the more to be regretted, as we see reason to 

 believe that " E 7 " must have corresponded pretty closely with the 

 stage now to be described. Semon notes that the embryonic area of 

 this egg was injured through an unfortunate accident. 



The general structural arrangements of the Monotreme ovum in its 

 early stages of development have been described and figured by 

 Semon. In all the stages dealt with by him, illustrating the develop- 

 ment from segmentation up to the first indications of gastrulation, the 

 yolk had retained its original arrangement as a tolerably coherent 

 solid or semi-solid spheroidal mass with alternating strata of white and 

 yellow yolk-spheres. As he states, however, the egg in the course of 

 further development increases considerably in size during its sojourn 

 in the uterus through absorption of fluid. By this process of fluid 

 absorption, the yolk-mass is disintegrated and its spherules dissemi- 

 nated throughout the interior of the growing blastodermic vesicle, 

 though many of them remain adherent to the deep surface of the 

 blastodermic membrane. 



This conversion of the solid or semi-solid yolk-mass into the fluid 

 contents of a large blastodermic vesicle renders the investigation of 

 the structure of the blastoderm, from the period of the commencement 

 of gastrulation up to the formation of a distinct embryo, an exceed- 

 ingly difficult one. The loss of Semon's "E 7 " may very likely be 

 attributable to such difficulties in the way of manipulation as we have 

 encountered in dealing with a delicate blastodermic membrane sur- 

 rounded by a thick, tough, and opaque shell as well as a vitelline 



* Semon, R., ' Zcol. Forschungsreisen,' &c, Bd. 2, Lief. 1 (1894). 



