50 ON A PLATYPUS EMBRYO, 



give rise to the appearance seen in surface view. The mesoderm 

 extends out as a continuous plate of uniform thickness beyond the 

 edge of the medullary plate, whereas the medullary plate itself 

 shows a very noticeable thickening as compared with that of the 

 other sections figured. 



The marked retardation in the formation of the medullary folds 

 and in the folding off of the embryo is one of the most character- 

 istic features of the embryo at this stage. This may be due, as 

 suggested by our friend Prof. J. T. Wilson, to the mechanical 

 effect of the rapid imbibition by the ovum of nutritive fluid 

 secreted by the uterine glands. 



The mature ovarian ovum, according to Caldwell,"^ measures 

 only 2*5 to 3 mm. in diameter. After the entrance of the ovum 

 into the Fallopian tube the shell membrane and proalbumen are 

 added externally to the vitelline membrane, and at the same time 

 the ovum is increasing in size by the absorption of fluid. The 

 youngest stages in our possession are eggs in which segmentation 

 has advanced to some extent ; they measure 5 mm. in diameter, 

 and possess a distinct and resistent shell membrane separated 

 from the vitelline membrane by a thin layer of proalbumen. As 

 development proceeds this layer of proalbumen is soon wholly 

 absorbed, and in the eggs under consideration the blastodermic 

 vesicle tightly distended with fluid fitted closely around the inner 

 surface of the fully formed shell, the vitelline membrane being no 

 longer recognisable. During the rapid imbibition of fluid by the 

 blastodermic vesicle, and the consequent increase in size of the 

 whole ovum, the wall of the vesicle including the embryonal area 

 is closely pressed against the surrounding vitelline and shell 

 membranes. It therefore seems reasonable to suppose that under 

 such conditions, only those structural features of the embryo are 

 produced which do not involve any upgrowths of the wall of the 

 vesicle. Once the definite shell is fully formed around the egg 

 and no possibility exists of its obtaining a further supply of 

 maternal nutritive material, the normal development of bodily 



* Phil. Trans. 1887. 



