738 



ON A PLATYPUS EMBRYO FROM THE INTRA 

 UTERINE EGG. 



By J. P. Hill, Demonstrator of Biology, axd C. J. Martin, 

 M.B., B.Sc. (LoND.), Demonstrator of Physiology, in the 

 University op Sydney. 



[Abslract.) 



The paper is based on the examination of two embiyos taken 

 from eggs just ready to be laid. The eggs, both exactly of the 

 same size, measured 18 by 13-5 mm., being somewhat larger than 

 those described by Caldwell. 



The embryo was found lying on the surface of a thin- walled 

 vesicle with its long axis corresponding to the long axis of the 

 egg. It measured 1 9 mm. in length from the anterior end of the 

 medullary plate to the posterior end of the primitive streak. The 

 vesicle on which the embryo lay consisted of two layers all over, 

 an outer layer of cubical ectoderm cells, and an inner layer of 

 large cells occupied almost entirely by yolk spheres — vitelline- 

 entoderm cells. Between the ectoderm and the vitelline-entoderm 

 the mesoderm extends round from a quarter to one-half of the 

 circumference of the vesicle in the posteiior region of the embryo. 

 The vesicle completely filled up the interior of the shell, and 

 contained a thin albuminous fluid. From the relatively very 

 early great lateral extension of the mesoderm, and from the 

 presence of a very distinct yolk-containing entoderm the vesicle 

 of the Platypus embryo of this stage is regarded as transitional 

 between the yolk-sac of Sauropsida and the typical mammalian 

 blastodermic vesicle. 



The embryo, with the exception of a slight head-fold, is quite 

 flat. Medullary folds are absent except in the anterior region of 

 the future fore-brain, where slight lateral upgrowths of the 

 medullaiy plate appear. The appearance of medullary folds in 

 this region is probably to be associated with the ^ery early 

 appearance of the optic grooves. 



